Fixes https://bugs.llvm.org/show_bug.cgi?id=31877.
Fixes https://github.com/google/sanitizers/issues/1155.
Enables exceptions in msan/tsan buid of libcxx, and in msan tests.
-fdepfile-entry stuff is a workaround for
https://reviews.llvm.org/D69290 (default blacklist missing from -MMD
output).