[CMake] add_llvm_library: don't use .imp suffix for import libraries on Windows (PR22334)

This was added in r188351 to fix a naming conflict between the
profile_rt-static and profile_rt-shared who both ended up in
lib/profile_rt.lib.

The change also affected other libraries (like libclang), and
users are reporting that they find it surprising that there's
no longer a libclang.lib. Since the profile_rt naming conflict
doesn't seem to exist any more, I think we can remove this.

Differential Revision: http://reviews.llvm.org/D7391

llvm-svn: 228049