Rationale:

- To do scope analysis based on PreParser, and use the result again when parsing
  later, PreParser and Parser need to produce the same Scopes and variable
  declarations in them.

- This is not the case for non-simple parameters: Parser creates an additional
  inner Scope where the declarations were, whereas PreParser does
  DeclareVariableName directly in the function Scope.

- So this CL fixes that by moving the Scope creation for non-simple parameters
  into ParserBase.

- As a side product (and a partial proof that this change makes sense),
  PreParser::ParseEagerFunctionBody is now gone.

BUG=v8:5516

Review-Url: https://codereview.chromium.org/2638333002
Cr-Commit-Position: refs/heads/master@{#42537}