[PlaceSafepoints] New attributes for patchable statepoints.

Summary:
This patch teaches the PlaceSafepoints pass about two `CallSite`
function attributes:

 * "statepoint-id": if the string value of this attribute can be parsed
   as an integer, then it is propagated to the ID parameter of the
   statepoint created.

 * "statepoint-num-patch-bytes": if the string value of this attribute
   can be parsed as an integer, then it is propagated to the `num patch
   bytes` parameter of the statepoint created.

This change intentionally does not assert on a malformed value for these
attributes, given that they're not "official" attributes.

Reviewers: reames, pgavlin

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9735

llvm-svn: 237286