[hwasan] Add a basic API.

Summary:
Add user tag manipulation functions:
  __hwasan_tag_memory
  __hwasan_tag_pointer
  __hwasan_print_shadow (very simple and ugly, for now)

Reviewers: vitalybuka, kcc

Subscribers: kubamracek, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D50746

llvm-svn: 339746