* Discard pattern versus symbol named `_`

- A discard pattern is not accepted at the top level in a switch statement. In that case `_` binds to a constant in scope with a warning WRN_CaseConstantNamedUnderscore that is included in a warning wave.
- A discard pattern is not accepted at the top level in an is-expression. In that case `_` binds to a type in scope with a warning WRN_IsTypeNamedUnderscore that is included in a warning wave.
- Elsewhere a pattern `_` is a discard without warning no matter what definition for `_` might be in scope.

Fixes #30650
See also #30846