It belongs there more logically. In addition, this is a pre-step needed
for preparsing the parameters of a preparsed function.

In addition, move the "subtract rest parameter from arity" logic from
Parser to (Pre)?ParserFormalParameters.

BUG=v8:5515

Review-Url: https://codereview.chromium.org/2414003002
Cr-Commit-Position: refs/heads/master@{#40258}