Couple of changes to the linker's dwarf test, including:

 - add some code to the DWARF tests inlining coverage to verify the
   call_file attribute attached to inlined routine DIEs. If function
   main.F is inlined into function main.G, we want to see that the
   call_file attribute in the inlined routine DIE for main.F is the
   same file as that reported for main.G.

 - fix a glitch with the way the DW_AT_decl_file attribute was
   being checked. The previous code relied on hard-coded indices
   into the line table files table, which is very brittle (since
   there is no requirement that files be ordered in any specific
   way). Instead, add machinery to look up the actual file string
   via the line table reader.

Change-Id: I44e71c69b6e676238cf4b805e7170de17b50939f
Reviewed-on: https://go-review.googlesource.com/c/go/+/196517
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>