[cmake] Allow testing against installed LLVM with no sources
[cmake] Allow testing against installed LLVM with no sources

Adjust the stand-alone build files to accept either CMake files from
LLVM_CMAKE_PATH or from LLVM_MAIN_SRC_DIR instead of requiring both.
This makes it possible to run libcxx tests on top of installed LLVM
and lit, without having to unpack a copy of LLVM sources. Furthermore,
it avoids adding duplicate paths.

Differential Revision: https://reviews.llvm.org/D25093

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@284583 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx