cmake: don't set LLVM_COMPILER_IS_GCC_COMPATIBLE when using clang-cl
cmake: don't set LLVM_COMPILER_IS_GCC_COMPATIBLE when using clang-cl

Tip-of-tree CMake has become clang-cl aware [1]. In this case,
CMAKE_CXX_COMPILER_ID will still be Clang, but MSVC will be true.

[1] See http://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=3d8356d4

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@192139 91177308-0d34-0410-b5e6-96231b3b80d8

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