[CMake] Use just basename when copying C++ ABI headers

This avoids duplicate directories when the filename includes path.

Fixes PR39145

Differential Revision: https://reviews.llvm.org/D52762

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@343753 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed