Clang: Enable exceptions explicitly

For some reason, clang 3.8.0 on Windows does not enable exceptions anymore,
which leads to parse errors in MSVC headers (reported upstream [1]).

With this change, we can finally parse main/mainwindow.cpp of a Qt Widgets
Application for a MSVC2015 Kit and libclang 3.8.0 without any error.

[1] https://llvm.org/bugs/show_bug.cgi?id=27324

Change-Id: I532ad4852a06318baf083d363378bc577b3c4309
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>

src/plugins/cpptools/compileroptionsbuilder.h

@@ -46,6 +46,7 @@ public:
 
     // Add options based on project part
     virtual void addTargetTriple();
+    virtual void enableExceptions();
     void addHeaderPathOptions();
     void addToolchainAndProjectDefines();
     virtual void addLanguageOption(ProjectFile::Kind fileKind);