[llvm] [cmake] Add possibility to use ChooseMSVCCRT.cmake when include LLVM library

Modify LLVMConfig to produce LLVM_USE_CRT variables in build-directory. It helps to set the same compiler debug options like in builded library.

Committed on behalf of @igorban (Igor)

Differential Revision: https://reviews.llvm.org/D67175

llvm-svn: 372610