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