cmake: Don't do the libstdc++ version check when clang simulates MSVC

cmake: Don't do the libstdc++ version check when clang simulates MSVC

If we're using clang-cl, that's a pretty good indication that we're
going to use MSVC's STL.

This simplifies the clang-cl ninja self-host configuration down to:
CC=clang-cl CXX=clang-cl cmake .. -GNinja

Modified version of zturner's patch:
Differential Revision: http://reviews.llvm.org/D7824

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230239 91177308-0d34-0410-b5e6-96231b3b80d8