Run Sourcekitd-tests in a non dispatch thread

Since Dispatch threads have a 64k stack size, tests were failing due to
blowing the stack on Windows. This runs the tests in thread with a
larger stack size. This fixes some 90ish sourcekit tests on Windows.

tools/SourceKit/lib/SwiftLang/SwiftASTManager.cpp

@@ -186,6 +186,15 @@ namespace SourceKit {
 
   void ASTUnit::Implementation::consumeAsync(SwiftASTConsumerRef ConsumerRef,
                                              ASTUnitRef ASTRef) {
+#if defined(_WIN32)
+	// Windows uses more up for stack space (why?) than macOS/Linux which
+	// causes stack overflows in a dispatch thread with 64k stack. Passing
+	// useDeepStack=true means it's given a _beginthreadex thread with an 8MB
+	// stack.
+	bool useDeepStack = true;
+#else
+	bool useDeepStack = false;
+#endif
     Queue.dispatch([ASTRef, ConsumerRef]{
       SwiftASTConsumer &ASTConsumer = *ConsumerRef;
 
@@ -197,7 +206,7 @@ namespace SourceKit {
         LOG_WARN_FUNC("did not find primary SourceFile");
         ConsumerRef->failed("did not find primary SourceFile");
       }
-    });
+    }, useDeepStack);
   }
 
   ASTUnit::ASTUnit(uint64_t Generation, std::shared_ptr<SwiftStatistics> Stats)