Clang: Check in translation unit if the file exists

We get in crashes in clang if we not check if the file exists.

Change-Id: I78a20507db6403247c02f8c1e69cd872e3e8f774
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/tools/clangbackend/ipcsource/translationunit.cpp

@@ -132,6 +132,7 @@ CXIndex TranslationUnit::index() const
 CXTranslationUnit TranslationUnit::cxTranslationUnit() const
 {
     checkIfNull();
+    checkIfFileExists();
     removeTranslationUnitIfProjectPartWasChanged();
     createTranslationUnitIfNeeded();
     reparseTranslationUnitIfFilesAreChanged();