Emit diagnostics for parsing disambiguation

[simanerush]

As a regular user of the Swift programming language, I get frustrated when I see diagnostics for the following cases dealing with cast destination types:

1. let _ = 1 as Int16 < 7
2. let _ = 1 as Int16 << 7

In both cases, I see error: expected type diagnostics, even though the context of the expression does not imply any presence of the type after <.

Proposed Solution: I would like to see more helpful diagnostics. For example, here is what Rust does in the same situation:

error: `<` is interpreted as a start of generic arguments for `i16`, not a shift
 --> <source>:3:16
  |
3 |     num as i16 << 7
  |                ^^ - interpreted as generic arguments
  |                |
  |                not interpreted as shift
  |
help: try shifting the cast value
  |
3 |     (num as i16) << 7
  |     +          +

Alternatives The alternative would be silencing the expected error diagnostics, and this was already attempted in my PR #60088. However, @CodaFi provided a great feedback which says,

This embeds a form of unbounded lookahead in the Swift type grammar. We should be careful committing to additional lookahead like this to disambiguate parses. This is strictly a change to the language and should probably be discussed on evolution.

I tend to agree with that, so therefore I think it's best to change the diagnostics in this case instead of proceeding with the alternative solution in my PR.

[CodaFi]

Diagnostic changes don't usually need to go through evolution since you're just using lookahead to pattern match the token stream but not actually changing the parse. This issue is a fine way to catalogue the feature request here. We should close out the other one in favor of this.

[simanerush]

Yes, thank you for clarification. I will close the other one!

[AnthonyLatsis]

@CodaFi Maybe we should keep the other one open so that we can close this one once we deal with the diagnostics QoI part?