[libcxx] Support runtimes and monorepo locations for tests

The test configuration support currently searches for libc++ sources
in <ROOT>/projects/libcxx. This change also additionally searches
<ROOT>/runtimes/libcxx (so called runtimes layout) and <ROOT>/libcxx
(monorepo layout).

This matches the logic we already use in CMake, for example:
https://github.com/llvm/llvm-project/blob/6fd4e7f/libcxx/CMakeLists.txt#L148

When the monorepo becomes the only supported layout in the future,
we can simplify this logic again.

Differential Revision: https://reviews.llvm.org/D57776

llvm-svn: 353600