swift234

[Sema] Use a TypeLoc for conversions with contextual types.

With a TypeLoc, we have a chance to offer diagnostics or even fix-its
to the contextual type, even though it's not represented by an
expression in the constraint system. This commit mostly just passes it
through, without attempting to use it anywhere or even pass a real
TypeLoc (with a valid TypeRepr).

(It does drop the contextual type parameter from
typeCheckExpressionShallow, since there were zero callers using it.)

No functionality change...yet.