[libcxx] Fix PR31402: map::__find_equal_key has undefined behavior.

Summary:
This patch fixes llvm.org/PR31402 by replacing `map::__find_equal_key` with `__tree::__find_equal`, which has already addressed the same undefined behavior.

Unfortunately I haven't been able to write a test case which causes the UBSAN diagnostic mentioned in the bug report. I can write tests which exercise the UB but for some reason they do not cause UBSAN to fail. Any help writing a test case would be appreciated.


Reviewers: mclow.lists, vsk, EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D28131

llvm-svn: 291087