[CodeGen] Support custom format of stack maps

Summary:
Add a hook to the GCMetadataPrinter for emitting stack maps in
custom format. The hook will be called at stack map generation
time. The default stack map format is used if there is no hook.

For this to be useful a few data structures and accessors are
exposed from the StackMaps class, so the custom printer can
access the stack map data.

This patch authored by Cherry Zhang <cherryyz@google.com>.

Reviewers: thanm, apilipenko, reames

Reviewed By: reames

Subscribers: reames, apilipenko, nemanjai, javed.absar, kbarton, jsji, llvm-commits

Differential Revision: https://reviews.llvm.org/D53892

llvm-svn: 347584