[hwasan] Fix top PC in error stack trace being off by 1 instruction.

Summary:
GetStackTrace treats top PC as a return address from an error reporting
function, and adjusts it down by 1 instruction. This is not necessary in
a signal handler, so adjust PC up to compensate.

Reviewers: kcc, vitalybuka, jfb

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D52802

llvm-svn: 343638