[libcxx] Move CMake file to src, avoid using globs

This addresses the longstanding FIXME and makes libc++ build more
similar to other runtimes.

Differential Revision: https://reviews.llvm.org/D61275

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@359656 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed