This is hopefully the last in a series of cleanup patches around
destructuring assignment. It simplifies the ParseAssignmentExpression
API, making the callers call CheckDestructuringElement() where appropriate.
CheckDestructuringElement has been further simplified to only emit the
errors that the parser depends on it emitting.

I've also beefed up the test coverage in test-parsing.cc to
handling all the destructuring flags being on, which caught an oddity
in how we disallow initializers in spreads in patterns (we need to treat
RewritableAssignmentExpressions as Assignments for the purpose of
error checking).

Finally, I added a few helper methods to ParserBase to handle a few
classes of expressions (assignments and literals-as-patterns).

Review URL: https://codereview.chromium.org/1696603002

Cr-Commit-Position: refs/heads/master@{#33961}