When providing scope information (containing the value of local
variables of live stack frames), decode the local variable names of all
functions in a wasm module and store this in the WasmDebugInfo
structure.
Use these names to actually name the reported locals, instead of using
the default names "param#<d>" and "local#<d>". These names are only used
as fallbacks for locals which were not assigned a name.

R=titzer@chromium.org,kozyatinskiy@chromium.org
BUG=v8:6245

Change-Id: Ibf7d30e392248ef5590177cd8b6329239b45e018
Reviewed-on: https://chromium-review.googlesource.com/548495
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46379}