[CMake] Let llvm_add_library(MODULE) check capability of loadable module.

On unsupported platforms, llvm_add_library(MODULE) doesn't create any targets.
Caller may be responsible to check and add extra target properties.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201320 91177308-0d34-0410-b5e6-96231b3b80d8