[TBDGen] Make enumeratePublicSymbols more functional

Instead of taking an out parameter, have it return
the set directly. Also coalesce the two overloads
into a single overload that takes a
`TBDGenDescriptor`.

Author: hamishknight

include/swift/TBDGen/TBDGen.h

@@ -14,6 +14,7 @@
 
 #include "llvm/ADT/Hashing.h"
 #include "llvm/ADT/StringSet.h"
+#include "swift/AST/TBDGenRequests.h"
 #include "swift/Basic/Version.h"
 #include <vector>
 
@@ -88,10 +89,7 @@ struct TBDGenOptions {
   }
 };
 
-void enumeratePublicSymbols(FileUnit *module, llvm::StringSet<> &symbols,
-                            const TBDGenOptions &opts);
-void enumeratePublicSymbols(ModuleDecl *module, llvm::StringSet<> &symbols,
-                            const TBDGenOptions &opts);
+llvm::StringSet<> getPublicSymbols(TBDGenDescriptor desc);
 
 void writeTBDFile(ModuleDecl *M, llvm::raw_ostream &os,
                   const TBDGenOptions &opts);

lib/FrontendTool/FrontendTool.cpp

@@ -1054,9 +1054,9 @@ static bool writeLdAddCFileIfNeeded(CompilerInstance &Instance) {
   }
   auto tbdOpts = Invocation.getTBDGenOptions();
   tbdOpts.LinkerDirectivesOnly = true;
-  llvm::StringSet<> ldSymbols;
   auto *module = Instance.getMainModule();
-  enumeratePublicSymbols(module, ldSymbols, tbdOpts);
+  auto ldSymbols =
+      getPublicSymbols(TBDGenDescriptor::forModule(module, tbdOpts));
   std::error_code EC;
   llvm::raw_fd_ostream OS(Path, EC, llvm::sys::fs::F_None);
   if (EC) {
@@ -1663,15 +1663,17 @@ static bool generateCode(CompilerInstance &Instance, StringRef OutputFilename,
                      OutputFilename, Instance.getStatsReporter());
 }
 
-static void collectLinkerDirectives(const CompilerInvocation &Invocation,
-                                    ModuleOrSourceFile MSF,
-                                    llvm::StringSet<> &Symbols) {
+static llvm::StringSet<>
+collectLinkerDirectives(const CompilerInvocation &Invocation,
+                        ModuleOrSourceFile MSF) {
   auto tbdOpts = Invocation.getTBDGenOptions();
   tbdOpts.LinkerDirectivesOnly = true;
-  if (MSF.is<SourceFile*>())
-    enumeratePublicSymbols(MSF.get<SourceFile*>(), Symbols, tbdOpts);
-  else
-    enumeratePublicSymbols(MSF.get<ModuleDecl*>(), Symbols, tbdOpts);
+  if (auto *SF = MSF.dyn_cast<SourceFile *>()) {
+    return getPublicSymbols(TBDGenDescriptor::forFile(SF, tbdOpts));
+  } else {
+    return getPublicSymbols(
+        TBDGenDescriptor::forModule(MSF.get<ModuleDecl *>(), tbdOpts));
+  }
 }
 
 static bool performCompileStepsPostSILGen(CompilerInstance &Instance,

lib/FrontendTool/TBD.cpp

@@ -139,9 +139,7 @@ bool swift::validateTBD(ModuleDecl *M,
                         const llvm::Module &IRModule,
                         const TBDGenOptions &opts,
                         bool diagnoseExtraSymbolsInTBD) {
-  llvm::StringSet<> symbols;
-  enumeratePublicSymbols(M, symbols, opts);
-
+  auto symbols = getPublicSymbols(TBDGenDescriptor::forModule(M, opts));
   return validateSymbolSet(M->getASTContext().Diags, symbols, IRModule,
                            diagnoseExtraSymbolsInTBD);
 }
@@ -150,9 +148,7 @@ bool swift::validateTBD(FileUnit *file,
                         const llvm::Module &IRModule,
                         const TBDGenOptions &opts,
                         bool diagnoseExtraSymbolsInTBD) {
-  llvm::StringSet<> symbols;
-  enumeratePublicSymbols(file, symbols, opts);
-
+  auto symbols = getPublicSymbols(TBDGenDescriptor::forFile(file, opts));
   return validateSymbolSet(file->getParentModule()->getASTContext().Diags,
                            symbols, IRModule,
                            diagnoseExtraSymbolsInTBD);

lib/TBDGen/TBDGen.cpp

@@ -1190,19 +1190,9 @@ GenerateTBDRequest::evaluate(Evaluator &evaluator,
   return std::make_pair(std::move(file), std::move(symbols));
 }
 
-void swift::enumeratePublicSymbols(FileUnit *file, StringSet &symbols,
-                                   const TBDGenOptions &opts) {
-  assert(symbols.empty() && "Additive symbol enumeration not supported");
-  auto &evaluator = file->getASTContext().evaluator;
-  auto desc = TBDGenDescriptor::forFile(file, opts);
-  symbols = llvm::cantFail(evaluator(GenerateTBDRequest{desc})).second;
-}
-void swift::enumeratePublicSymbols(ModuleDecl *M, StringSet &symbols,
-                                   const TBDGenOptions &opts) {
-  assert(symbols.empty() && "Additive symbol enumeration not supported");
-  auto &evaluator = M->getASTContext().evaluator;
-  auto desc = TBDGenDescriptor::forModule(M, opts);
-  symbols = llvm::cantFail(evaluator(GenerateTBDRequest{desc})).second;
+StringSet swift::getPublicSymbols(TBDGenDescriptor desc) {
+  auto &evaluator = desc.getParentModule()->getASTContext().evaluator;
+  return llvm::cantFail(evaluator(GenerateTBDRequest{desc})).second;
 }
 void swift::writeTBDFile(ModuleDecl *M, llvm::raw_ostream &os,
                          const TBDGenOptions &opts) {