[CMake] Get libcxx building under LLVM/runtimes

Summary:
The new LLVM runtimes build directory requires some basic conventions across the runtime projects. These changes make libcxx build under the runtimes subdirectory. The general idea of the changes is that the runtimes subdirectory requires some conventions to be consistent across runtime projects.

I expect to have a few more small patches that build on this to tie up check targets and other things useful in development workflows.

Summary of changes in this patch:

* Renamed variable LLVM_CONFIG -> LLVM_CONFIG_PATH
* Renamed variable LIBCXX_BUILT_STANDALONE -> LIBCXX_STANDALONE_BUILD
* Add an include of AddLLVM in the tests subdirectory for add_lit_testsuite.

Reviewers: EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D23696

llvm-svn: 279151