[CMake]Use LLVM_LIBRARY_DIR for lib path.

Summary:
This makes sure the correct lib path is being used when `CMAKE_CFG_INTDIR` or
`LLVM_LIBDIR_SUFFIX` is set.

Reviewers: beanz

Subscribers: mgorny, srhines, pirama, llvm-commits

Differential Revision: https://reviews.llvm.org/D35318

llvm-svn: 307985