[CMake] Support for monorepo layout

Support finding libcxx and libunwind sources in monorepo style layout.

Differential Revision: https://reviews.llvm.org/D45269

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@329208 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed