HWASan exception support.

Summary:
Adds a call to __hwasan_handle_vfork(SP) at each landingpad entry.

Reusing __hwasan_handle_vfork instead of introducing a new runtime call
in order to be ABI-compatible with old runtime library.

Reviewers: pcc

Subscribers: kubamracek, hiraditya, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D61968

llvm-svn: 360959