A slightly clearer diagnostic when misusing const
Fixes#79598
This produces the following diagnostic:
"expected one of `>`, a const expression, lifetime, or type, found keyword `const`"
Instead of the previous, more confusing:
"expected one of `>`, const, lifetime, or type, found keyword `const`"
This might not be completely clear as some users might not understand what a const expression is, but I do believe this is an improvement.
bb0d481b5a