Improve error message when '=' is missing in {ASAN,...}_OPTIONS.

Summary:
It's handling isses as described here:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89832

Patch by Martin Liška.

Reviewers: kcc, vitalybuka

Reviewed By: vitalybuka

Subscribers: cryptoad, kubamracek

Differential Revision: https://reviews.llvm.org/D59876

llvm-svn: 363480