Function names are optional in wasm and might not be present for most
functions. Instead of storing an empty name with each function, this
change loads names, if present, on first access of the name.

This also fixes an inconsistency with streaming compilation. Under
streaming compilation, functions are compiled before parsing the name
section. Hence, they always received an empty name. With this change,
assignment of names is typically deferred until the whole module was
parsed.

Bug: chromium:820291
Change-Id: I86d76aa40b7c45897d152725547795c8b6b9b9ba
Reviewed-on: https://chromium-review.googlesource.com/955647
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51909}