[CodeGen] Expose some data types and accessors from StackMaps

Summary:
This is for supporting custom stack map formats, where the
custom printer can access the stack map data.

Patch by Cherry Zhang <cherryyz@google.com>.

Related: https://reviews.llvm.org/D53892

Reviewers: thanm, apilipenko

Reviewed By: apilipenko

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D54224

llvm-svn: 347061