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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@230239 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,dslab-epfl/asap,GPUOpen-Drivers/llvm,llvm-mirror/llvm,dslab-epfl/asap,GPUOpen-Drivers/llvm,apple/swift-llvm,dslab-epfl/asap,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,dslab-epfl/asap,GPUOpen-Drivers/llvm,apple/swift-llvm,dslab-epfl/asap,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,dslab-epfl/asap,apple/swift-llvm,llvm-mirror/llvm,dslab-epfl/asap,apple/swift-llvm