Invert dependency between `rustc_error_messages` and `rustc_ast*`.

`rustc_error_messages` currently depends on
`rustc_ast`/`rustc_ast_pretty`. This is odd, because
`rustc_error_messages` feels like a very low-level module but
`rustc_ast`/`rustc_ast_pretty` do not.

The reason is that a few AST types impl `IntoDiagArg` via
pretty-printing. `rustc_error_messages` can define `IntoDiagArg` and
then impl it for the AST types. But if we invert the dependency we hit
a problem with the orphan rule: `rustc_ast` must impl `IntoDiagArg`
for the AST types, but that requires calling pretty-printing code which
is in `rustc_ast_pretty`, a downstream crate.

This commit avoids this problem by just removing the `IntoDiagArg` impls
for these AST types. There aren't that many of them, and we can just use
`String` in the relevant error structs and use the pretty printer in the
downstream crates that construct the error structs. There are plenty of
existing examples where `String` is used in error structs.

There is now no dependency between `rustc_ast*` and
`rustc_error_messages`.