CMake: Fix stand-alone clang builds since r353268
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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@354417 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm