[StackMaps] Add location size to llvm-readobj -stackmap output

The size field of a location can be different for each entry, so it is useful to have this displayed in the output of llvm-readobj -stackmap. Below is an example of how the output would look:

Record ID: 2882400000, instruction offset: 16
   3 locations:
     #1: Constant 1, size: 8
     #2: Constant 2, size: 8
     #3: Constant 3, size: 8
   0 live-outs: [ ]

Patch By: jacob.hughes@kcl.ac.uk (with heavy modification by me)
Differential Revision: https://reviews.llvm.org/D59169

llvm-svn: 358324