Compiler backends get their language mode from the current
function, but should instead be deriving it from the current scope.
This allows proper handling of the always-strictness of class declarations
and expressions, and in particular the treatment of 'eval' calls in an
extends clause as a strict eval.

Also fix the parser's RecordEvalCall logic to only reach out to the
DeclarationScope in sloppy mode, which fixes the strange case of a
sloppy function thinking it contains a sloppy eval when in fact
it contains only a strict eval.

BUG=v8:4970
LOG=n

Review-Url: https://codereview.chromium.org/1931003003
Cr-Commit-Position: refs/heads/master@{#36001}