There was an inconsistency where in the case of array repeat
expressions, we forgot to eat the `const` keyword of mgca const blocks.
Thus, we ended up parsing them as an anon const containing an inline
const, instead of simply an anon const as they should be.
This commit fixes that issue and also makes the parsing for mgca const
blocks more consistent and simpler in general. For example, we avoid
doing a lookahead check to ensure there's a curly brace coming. There
should always be one after a `const` keyword in an expression context,
and that's handled by the mgca const block parsing function.
cyrgani