Make headers available in the build directory

This essentially re-does r194825 and makes it possible to run clang
with libc++ without having to install it, even if you don't have any
version of libc++ installed in /usr/.

This behaviour broke in r210577/r211629, which fixed pr18681.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@220489 91177308-0d34-0410-b5e6-96231b3b80d8