Minor fix in ToolChainTest.cpp to allow user defined GCC toolchain.

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