[CMake] Clang: Don't use object libraries with Xcode

Undoes some of the effects of r360946 when using the Xcode CMake
generator---it doesn't handle object libraries correctly at all.
Attempts to still honor BUILD_SHARED_LIBS for Xcode, but I didn't
actually test it. Should have no effect on non-Xcode generators.

https://reviews.llvm.org/D68430

llvm-svn: 373769