Without this fix, AssignmentExpressions that happen to be arrow functions
would lead to unbalanced Enter/Leave calls on the fni_, causing thrashing
while trying to infer function names. Symptoms include slow parsing
or OOM (when we create too many AstConsStrings).

To try to keep this from happening in the future, added an RAII helper
class to handle Entering/Leaving FNI state.

The included regression test crashes on my workstation without the patch.
Note that it's too slow in debug mode (as well as under TurboFan),
so I've skipped it there.

BUG=v8:4595
LOG=y

Review URL: https://codereview.chromium.org/1507283003

Cr-Commit-Position: refs/heads/master@{#32768}