Clang: Disable internal threads in libclang

We already run all operations in threads to avoid blocking, there is no
need to start more threads within libclang. Before this change, a
reparse would trigger 3 threads to start:
  1. clangbackend: Utils::runAsync() for the job
  2. libclang-internal: thread for reparse
  3. libclang-internal: thread for annotating tokens (highlighting)

Ensure that we use the same stack size for our threads as libclang was
doing internally. C++ parsers usually have higher stack size
requirements.

Change-Id: I2f67602ddfbf77ea2c69144b56acf64ba08041f6
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/tools/clangbackend/source/clangasyncjob.h

@@ -58,7 +58,9 @@ public:
                          &QFutureWatcher<Result>::finished,
                          onFinished);
 
-        const QFuture<Result> future = Utils::runAsync(m_runner);
+        // Use 16MB stack size as clang_annotateTokens() would with an internal thread.
+        const Utils::StackSizeInBytes stackSize = 1024 * 1024 * 16;
+        const QFuture<Result> future = Utils::runAsync(stackSize, m_runner);
         m_futureWatcher.setFuture(future);
 
         return future;