Both the is_const and declaration_scope fields can be reliably derived
from the mode field. needs_init cannot be, unfortunately, due to the
special case of CONST in for loops.

Also inline the sole remaining non-trivial caller of
Parser::DeclarationScope(VariableMode).

Review URL: https://codereview.chromium.org/1487603003

Cr-Commit-Position: refs/heads/master@{#32536}