[libc++] Take 2: Implement CTAD for map and multimap

This is a re-application of r362986 (which was reverted in r363688) with fixes
for the issue that caused it to be reverted.

Thanks to Arthur O'Dwyer for the patch.

Differential Revision: https://reviews.llvm.org/D58587

llvm-svn: 363968
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: f2f7d72f0052b169433790d79e06b5160325c538
8 files changed