Make libcxx tests work when llvm sources are not present.
Make libcxx tests work when llvm sources are not present.

Despite a strong CMake warning that this is an unsupported
libcxx build configuration, some bots still rely on being
able to check out lit and libcxx independently with no
LLVM sources, and then run lit against libcxx.

A previous patch broke that workflow, so this is making it work
again.  Unfortunately, it breaks generation of the llvm-lit
script for libcxx, but we will just have to live with that until
a solution is found that allows libcxx to make more use of
llvm build pieces.  libcxx can still run tests by using the
ninja check target, or by running lit.py directly against the
build tree or source tree.

Differential Revision: https://reviews.llvm.org/D38057

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@313763 91177308-0d34-0410-b5e6-96231b3b80d8

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