[CodeCompletion] Fix crashes in TypeCheckASTNodeAtLocRequest

  receive {
    switch <HERE>
  }

In the current Parser implementation, if there's a error in 'switch'
subject, the 'SwitchStmt' is not placed in the parsed AST.

When the type checker searches AST node at the location, it used to find
'receive { ... }' because that is the *innermost* node in the AST that
contains the loc. But the decl context is the closure. This behavior used
to cause crashes because 'receive { ... }' is re-typechecked.

For solution, when finding the innermost AST node, clear the found node
in parent context before walking into brace statement.

rdar://problem/69246891

Author: rintaro

lib/Sema/TypeCheckStmt.cpp

@@ -1841,12 +1841,23 @@ bool TypeCheckASTNodeAtLocRequest::evaluate(Evaluator &evaluator,
 
     std::pair<bool, Stmt *> walkToStmtPre(Stmt *S) override {
       if (auto *brace = dyn_cast<BraceStmt>(S)) {
+        auto braceCharRange = Lexer::getCharSourceRangeFromSourceRange(
+            SM, brace->getSourceRange());
+        // Unless this brace contains the loc, there's nothing to do.
+        if (!braceCharRange.contains(Loc))
+          return {false, S};
+
+        // Reset the node found in a parent context.
+        if (!brace->isImplicit())
+          FoundNode = nullptr;
+
         for (ASTNode &node : brace->getElements()) {
           if (SM.isBeforeInBuffer(Loc, node.getStartLoc()))
             break;
 
           // NOTE: We need to check the character loc here because the target
-          // loc can be inside the last token of the node. i.e. interpolated string.
+          // loc can be inside the last token of the node. i.e. interpolated
+          // string.
           SourceLoc endLoc = Lexer::getLocForEndOfToken(SM, node.getEndLoc());
           if (SM.isBeforeInBuffer(endLoc, Loc) || endLoc == Loc)
             continue;
@@ -1858,8 +1869,9 @@ bool TypeCheckASTNodeAtLocRequest::evaluate(Evaluator &evaluator,
 
           // Walk into the node to narrow down.
           node.walk(*this);
-        }
 
+          break;
+        }
         // Already walked into.
         return {false, nullptr};
       }

validation-test/IDE/crashers_2_fixed/rdar69246891.swift

@@ -1,4 +1,5 @@
 // RUN: %target-swift-ide-test -code-completion -code-completion-token=COMPLETE -source-filename=%s
+// RUN: %target-swift-ide-test -code-completion -code-completion-token=COMPLETE_2 -source-filename=%s
 
 class MyCls {
   public init(body: (Int) throws -> Void) {}
@@ -10,3 +11,11 @@ func foo() {
     }
   }
 }
+
+func receive(fn: () -> throws Void) {}
+
+func test2() {
+  receive {
+    switch #^COMPLETE_2^#
+  }
+}