[libcxx] Capture configuration information when installing the libc++ headers
This patch is a successor to D11963. However it has changed dramatically and I felt it would be best to start a new review thread.
Please read the design documentation added in this patch for a description of how it works.
Reviewers: mclow.lists, danalbert, jroelofs, EricWF
Subscribers: vkalintiris, rnk, ed, espositofulvio, asl, eugenis, cfe-commits
Differential Revision: http://reviews.llvm.org/D13407