[CMake] add_llvm_library: Specify explicit suffix .imp to import library to avoid a warning between profile_rt-static and profile_rt-shared with lib/profile_rt.lib.

FIXME: It seems MS version of profile_rt.dll doesn't contain any export symbols.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188351 91177308-0d34-0410-b5e6-96231b3b80d8