[hwasan] Add __hwasan_handle_longjmp.

Summary:
A callback to annotate longjmp-like code.

Unlike __asan_handle_no_return, in hwasan we can not conservatively
"unpoison" the entire thread stack, because there is no such thing as
unpoisoned memory. Pointer and memory tags must always match.

Reviewers: vitalybuka, kcc

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D50752

llvm-svn: 339935