Don't produce relocations for a difference in a section with no symbols.

We were producing a relocation for
----------------
.section foo,bar
La:
Lb:
 .long   La-Lb
--------------

but not for

---------------------
  .section foo,bar
zed:
La:
Lb:
 .long   La-Lb
----------------

This patch handles the case where both fragments are part of the first atom
in a section and there is no corresponding symbol to that atom.

This fixes pr21328.
Move cross-unit DIE caching to the DwarfFile level, so it doesn't interfere with fission-gmlt data and produce skeleton<>full unit cross referencing.
Provide gmlt-like inline scope information in the skeleton CU to facilitate symbolication without needing the .dwo files

Clang -gsplit-dwarf self-host -O0, binary increases by 0.0005%, -O2,
binary increases by 25%.

A large binary inside Google, split-dwarf, -O0, and other internal flags
(GDB index, etc) increases by 1.8%, optimized build is 35%.

The size impact may be somewhat greater in .o files (I haven't measured
that much - since the linked executable -O0 numbers seemed low enough)
due to relocations. These relocations could be removed if we taught the
llvm-symbolizer to handle indexed addressing in the .o file (GDB can't
cope with this just yet, but GDB won't be reading this info anyway).
Also debug_ranges could be shared between .o and .dwo, though ideally
debug_ranges would get a schema that could used index(+offset)
addressing, and move to the .dwo file, then we'd be back to sharing
addresses in the address pool again.

But for now, these sizes seem small enough to go ahead with this.

Verified that no other DW_TAGs are produced into the .o file other than
subprograms and inlined_subroutines.