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/libs/clangsupport/processcreator.h

@@ -29,6 +29,7 @@
 
 #include "processhandle.h"
 
+#include <utils/environment.h>
 #include <utils/temporarydirectory.h>
 
 #include <QStringList>
@@ -51,6 +52,7 @@ public:
     void setTemporaryDirectoryPattern(const QString &temporaryDirectoryPattern);
     void setProcessPath(const QString &m_processPath);
     void setArguments(const QStringList &m_arguments);
+    void setEnvironment(const Utils::Environment &environment);
     void setObserver(QObject *m_observer);
 
     std::future<QProcessUniquePointer> createProcess() const;
@@ -72,6 +74,7 @@ private:
     QString m_processPath;
     QString m_temporaryDirectoryPattern;
     QStringList m_arguments;
+    Utils::Environment m_environment;
     QObject *m_observer = nullptr;
 };