Clang: Avoid consuming gcc internal include paths

Given the (default) include paths of GCC, e.g.

 /usr/include/c++/7
 /usr/include/x86_64-linux-gnu/c++/7
 /usr/include/c++/7/backward
 /usr/lib/gcc/x86_64-linux-gnu/7/include
 /usr/local/include
 /usr/lib/gcc/x86_64-linux-gnu/7/include-fixed
 /usr/include/x86_64-linux-gnu
 /usr/include

discard gcc-internal paths like /usr/lib/gcc/x86_64-linux-gnu/7/include
as they are not relevant for clang and even confuse it with regard to
 #include_next.

Paths below the gcc install dir are considered as gcc-internal. The
install dir is queried with

  $ gcc -print-search-dirs

Some GCC distributions, like MinGW, ship the standard library headers in
the install dir. Ensure to not discard these.

Fixes: QTCREATORBUG-22898
Change-Id: Ia85258fb01b72ad073e71390e003fe8268e3b01f
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/cpptools/projectpart.h

@@ -42,6 +42,7 @@
 #include <cplusplus/Token.h>
 
 #include <utils/cpplanguage_details.h>
+#include <utils/fileutils.h>
 
 #include <QString>
 #include <QSharedPointer>
@@ -108,6 +109,7 @@ public:
     bool isMsvc2015Toolchain = false;
     QString toolChainTargetTriple;
     ToolChainWordWidth toolChainWordWidth = WordWidth32Bit;
+    Utils::FilePath toolChainInstallDir;
     ProjectExplorer::WarningFlags warningFlags = ProjectExplorer::WarningFlags::Default;
 
     // Misc