Rework Runtime::FunctionForName to take a c-string instead of a v8::String
so that the parser can parse native syntax runtime calls without doing
on-the-fly internalization.

Also adds a c-string variant of IntrinsicIndexForName for the same reasons.

BUG=v8:5215,chromium:634953

Review-Url: https://codereview.chromium.org/2324803002
Cr-Commit-Position: refs/heads/master@{#39346}