Remove special tokens for base type names (PHP itself doesn't
have them either in their AST). Unfortunately, keeping them,
while cleaner, is also a lot more complicated and would leave
us with double the amount of FIRST/FIRST conflicts in the grammar.
(Mostly when NamespacedIdentifiers would conflict with base types
in typehints. Think 'String\Stringhandler $foo' vs 'string $bar'.

Removing the tokens essentially immediately resolves the bug,
with the remaining work just being error handling and a bit of
cleanup.

BUG: 392759