ast_validation: forbid "nonstandard" literal patterns
Since #42886, macros can create "nonstandard" PatKind::Lit patterns,
that contain path expressions instead of the usual literal expr. These
can cause trouble, including ICEs.
We *could* map these nonstandard patterns to PatKind::Path patterns
during HIR lowering, but that would be much effort for little gain, and
I think is too risky for beta. So let's just forbid them during AST
validation.
Fixes#43250.
beta-nominating because regression.
r? @eddyb
bae4fafdfb