[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

llvm-svn: 284583