[mir] Serialize DILocation inline when not possible to use a metadata reference

Summary:
Sometimes MIR-level passes create DILocations that were not present in the
LLVM-IR. For example, it may merge two DILocations together to produce a
DILocation that points to line 0.

Previously, the address of these DILocations were printed which prevented the
MIR from being read back into LLVM. With this patch, DILocations will use
metadata references where possible and fall back on serializing them inline like so:
    MOV32mr %stack.0.x.addr, 1, _, 0, _, %0, debug-location !DILocation(line: 1, scope: !15)

Reviewers: aprantl, vsk, arphaman

Reviewed By: aprantl

Subscribers: probinson, llvm-commits

Tags: #debug-info

Differential Revision: https://reviews.llvm.org/D55243

llvm-svn: 349035