Just a refactoring to make clear that we are talking about function
names. Note that there are also names for locals inside functions,
which we currently don't use.

Drive-by: Remove style-guide violation by {WasmModule::names_} field.

R=mstarzinger@chromium.org

Bug: v8:7754
Change-Id: I9c47ea01893f128e1716be01032adfaf006ae28a
Reviewed-on: https://chromium-review.googlesource.com/1118271Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54105}