Clang: Explicitly set the target triple

As for the analyzer, this makes us independent of the default triple and will
most likely reduce the maintenance - e.g. the target implies certain internal
command line arguments, we will profit from added ones.

This fixes parsing of mingw headers with the clang code model.

Change-Id: I722b981125a80fac5f62a7af40a83ecdd7bbf811
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>

src/plugins/cpptools/projectpart.h

@@ -104,6 +104,7 @@ public: // fields
     QByteArray projectDefines;
     QByteArray toolchainDefines;
     Core::Id toolchainType;
+    QString targetTriple;
     ProjectPartHeaderPaths headerPaths;
     QStringList precompiledHeaders;
     LanguageVersion languageVersion;