If the user configured clang with a custom GCC toolchain that will take precedence on what the ToolChainTest.cpp expects to evaluate. 
This is fixed here by passing --gcc-toolchain= to the driver, in order to override any user defined GCC toolchain.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@251459 91177308-0d34-0410-b5e6-96231b3b80d8

This is what most people want anyways. Clang -cc1's main() will override
this but for other tools this is the most sensible default and avoids
some work.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@250164 91177308-0d34-0410-b5e6-96231b3b80d8

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@249846 91177308-0d34-0410-b5e6-96231b3b80d8

This lets a VFSified driver actually validate the GCC paths.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@249829 91177308-0d34-0410-b5e6-96231b3b80d8

There are still some loose ends here but it's sufficient so we can detect
GCC headers that are inside of a VFS.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@249556 91177308-0d34-0410-b5e6-96231b3b80d8