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/projectexplorer/rawprojectpart.cpp

@@ -184,6 +184,7 @@ ToolChainInfo::ToolChainInfo(const ToolChain *toolChain,
         wordWidth = toolChain->targetAbi().wordWidth();
         targetTriple = toolChain->originalTargetTriple();
         extraCodeModelFlags = toolChain->extraCodeModelFlags();
+        installDir = toolChain->installDir();
 
         // ...and save the potentially expensive operations for later so that
         // they can be run from a worker thread.