hwasan: Teach the runtime to identify the local variable being accessed in UAR reports.

Each function's PC is recorded in the ring buffer. From there we can access
the function's local variables and reconstruct the tag of each one with the
help of the information printed by llvm-symbolizer's new FRAME command. We
can then find the variable that was likely being accessed by matching the
pointer's tag against the reconstructed tag.

Differential Revision: https://reviews.llvm.org/D63469

llvm-svn: 364607