[lldb][ClangExpressionParser] Reset DiagnosticManager before we create persistent variables (llvm#160074)

Here's an example crash that we've seen sporadically over the years:
```
0   libsystem_kernel.dylib        	       0x19d392388 __pthread_kill + 8
1   libsystem_pthread.dylib       	       0x19d3cb88c pthread_kill + 296
2   libsystem_c.dylib             	       0x19d29cd04 raise + 32
3   LLDB                          	       0x112cbbc94 SignalHandler(int, __siginfo*, void*) + 324
4   libsystem_platform.dylib      	       0x19d4056a4 _sigtramp + 56
5   LLDB                          	       0x110dcbd38 clang::TextDiagnosticPrinter::HandleDiagnostic(clang::DiagnosticsEngine::Level, clang::Diagnostic const&) + 1216
6   LLDB                          	       0x110dcbd38 clang::TextDiagnosticPrinter::HandleDiagnostic(clang::DiagnosticsEngine::Level, clang::Diagnostic const&) + 1216
7   LLDB                          	       0x10de12128 ClangDiagnosticManagerAdapter::HandleDiagnostic(clang::DiagnosticsEngine::Level, clang::Diagnostic const&) + 332
8   LLDB                          	       0x1121eb3dc clang::DiagnosticIDs::ProcessDiag(clang::DiagnosticsEngine&) const + 200
9   LLDB                          	       0x1121e67a0 clang::DiagnosticsEngine::EmitCurrentDiagnostic(bool) + 100
10  LLDB                          	       0x111d766cc IsStructurallyEquivalent(clang::StructuralEquivalenceContext&, clang::FieldDecl*, clang::FieldDecl*, clang::QualType) + 1568
11  LLDB                          	       0x111d71b54 IsStructurallyEquivalent(clang::StructuralEquivalenceContext&, clang::RecordDecl*, clang::RecordDecl*) + 2076
12  LLDB                          	       0x111d6e448 clang::StructuralEquivalenceContext::Finish() + 204
13  LLDB                          	       0x111d6e1e0 clang::StructuralEquivalenceContext::IsEquivalent(clang::Decl*, clang::Decl*) + 32
14  LLDB                          	       0x111d3b788 clang::ASTNodeImporter::IsStructuralMatch(clang::Decl*, clang::Decl*, bool, bool) + 168
15  LLDB                          	       0x111d404e0 clang::ASTNodeImporter::VisitFunctionDecl(clang::FunctionDecl*) + 1300
16  LLDB                          	       0x111d5cae0 clang::ASTImporter::ImportImpl(clang::Decl*) + 24
17  LLDB                          	       0x10ddf30bc lldb_private::ClangASTImporter::ASTImporterDelegate::ImportImpl(clang::Decl*) + 308
18  LLDB                          	       0x111d48140 clang::ASTImporter::Import(clang::Decl*) + 984
19  LLDB                          	       0x10ddee9dc lldb_private::ClangASTImporter::CopyDecl(clang::ASTContext*, clang::Decl*) + 100
20  LLDB                          	       0x10ddfab40 lldb_private::ClangASTSource::FindExternalLexicalDecls(clang::DeclContext const*, llvm::function_ref<bool (clang::Decl::Kind)>, llvm::SmallVectorImpl<clang::Decl*>&) + 1692
21  LLDB                          	       0x111e1cb84 clang::DeclContext::LoadLexicalDeclsFromExternalStorage() const + 180
22  LLDB                          	       0x111e1df50 clang::DeclContext::buildLookup() + 204
23  LLDB                          	       0x111e1dcf4 clang::DeclContext::makeDeclVisibleInContextWithFlags(clang::NamedDecl*, bool, bool) + 504
24  LLDB                          	       0x111d3b01c clang::ASTNodeImporter::ImportDeclContext(clang::DeclContext*, bool) + 724
25  LLDB                          	       0x111d62d10 clang::ASTImporter::ImportDefinition(clang::Decl*) + 428
26  LLDB                          	       0x10ddf1cb0 lldb_private::ClangASTImporter::ASTImporterDelegate::ImportDefinitionTo(clang::Decl*, clang::Decl*) + 524
27  LLDB                          	       0x10ddef3c8 (anonymous namespace)::CompleteTagDeclsScope::~CompleteTagDeclsScope() + 616
28  LLDB                          	       0x10ddef6dc lldb_private::ClangASTImporter::DeportDecl(clang::ASTContext*, clang::Decl*) + 436
29  LLDB                          	       0x10ddec3dc lldb_private::ASTResultSynthesizer::CommitPersistentDecls() + 236
30  LLDB                          	       0x10de1091c lldb_private::ClangExpressionParser::ParseInternal(lldb_private::DiagnosticManager&, clang::CodeCompleteConsumer*, unsigned int, unsigned int) + 2292
31  LLDB                          	       0x10de21238 lldb_private::ClangUserExpression::TryParse(lldb_private::DiagnosticManager&, lldb_private::ExecutionContext&, lldb_private::ExecutionPolicy, bool, bool) + 328
...
```

Here `ASTImporter::IsStructurallyEquivalent` is trying to emit a
diagnostic using `ClangExpressionParser`'s
`ClangDiagnosticManagerAdapter`. But `TextDiagnosticPrinter::TextDiag`
seems to be `nullptr`. This can only happen when we call
`HandleDiagnostic` on `ClangDiagnosticManagerAdapter` after we called
`EndSourceFile`. Specifically, it looks like when moving a type from
`Expression` AST to `Scratch` AST (in `CommitPersistentDecls`),
something went wrong, so the ASTImporter tried to emit a diagnostic, but
we already called `EndSourceFile` at that point.

This patch moves the call to `ResetManager` to before
`CommitPersistentDecls`, so if we called `HandleDiagnostic`, we would
correctly short-circuit out of it. This seems to have been intended
anyway based on this comment:
https://github.com/llvm/llvm-project/blob/cecdff92838f3c049548e3445a15d8c9c7a49205/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp#L200-L204

But something must have broken that during a refactor.

I'm not 100% sure how best to test this because we need a scenario where
moving a type into the scratch AST fails, but the expression itself
succeeded.

rdar://159647906

Author: Michael137

lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp

@@ -1307,6 +1307,10 @@ ClangExpressionParser::ParseInternal(DiagnosticManager &diagnostic_manager,
   m_compiler->setSema(nullptr);
 
   adapter->EndSourceFile();
+  // Creating persistent variables can trigger diagnostic emission.
+  // Make sure we reset the manager so we don't get asked to handle
+  // diagnostics after we finished parsing.
+  adapter->ResetManager();
 
   unsigned num_errors = adapter->getNumErrors();
 
@@ -1322,8 +1326,6 @@ ClangExpressionParser::ParseInternal(DiagnosticManager &diagnostic_manager,
     type_system_helper->CommitPersistentDecls();
   }
 
-  adapter->ResetManager();
-
   return num_errors;
 }