Permit Coercions in Type Ascriptions

[b-naber]

We want to permit coercions of type ascriptions inside coercion sites. To implement this we introduce a TypeAscriptionCtxt, which we pass inside an Expectation down to sub-expressions. Once we encounter a coercion site, we set TypeAscriptionCtxt::Coercion in the Expectation and propagate this down into check_expr_with_expectation. Whenever we encounter a type ascription and TypeAscriptionCtxt::Coercion is set, we coerce the type ascription.

Since not all sites, in which check_expr_coercable_to_type calls originate, are coercion sites (e.g. ExprKind::Assign or some methods in op.rs use check_expr_coercable_to_type), we currently also pass a TypeAscriptionCtxt to check_expr_coercable_to_type. Exposing TypeAscriptionCtxt throughout typeck::check is probably bad, I'm not sure what's the best way to hide the Ctxt, maybe we could call different variants of 'check_expr_coercable_to_type' based on whether the site at which these calls originate is an actual coercion site or not?

Closes #78248

r? @nikomatsakis