[libc++] Add a CI job running MSAN

[libc++] Add a CI job running MSAN

For some reason, we've been going without a MSAN CI job, even though
even run-buildbot defined a generic-msan job. This must have been an
oversight that went unnoticed. Thanks to @EricWF for the catch.

Differential Revision: https://reviews.llvm.org/D120851