[parser cleanup] Use BlockState consistently in Parser

This avoids manual, fragile assignment to Parser::scope_. And once all such
assignments are removed, BlockState might become much more valuable as
a place to reset scope-related state.

Also remove unnecessary nullptr checks after calls to FinalizeBlockScope.

Review URL: https://codereview.chromium.org/1583453003

Cr-Commit-Position: refs/heads/master@{#33313}