CMake: Fix stand-alone clang builds since r353268

Summary:
Handle the case where LLVM_MAIN_SRC_DIR is not set and also use
LLVM_CMAKE_DIR for locating installed cmake files rather than
LLVM_CMAKE_PATH.

Reviewers: phosek, andrewrk, smeenai

Reviewed By: phosek, andrewrk, smeenai

Subscribers: mgorny, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D58204