[CS] Diagnose misuse of CheckedCastExpr with ~=

[jameesbrown]

Issue

When applying unwrap operator to 'as' or using 'is' in a type-cast pattern (specifically one that binds a value), the diagnostic for the resulting error could provide more guidance and suggest a fix-it.

Example with current diagnostic:

let maybeInt: Any = 1
if case let intValue as? Int = maybeInt {}
            `- error: pattern variable binding cannot appear in an expression

Fix

1. Added diagnostic invalid_cast_in_pattern and an associated fix-it to remove !/? or replace 'is' with 'as'.
2. Modified test/Constraints/rdar106598067.swift to account for the new diagnostic.

Example with new diagnostic:

if case let intValue as? Int = maybeInt {}
            ~~~~~~~~ ^ ~~~~~ 
                   //`- error: cannot conditionally downcast in a type-casting pattern
                   // - fix-it: remove '?'
if case let intValue as! Int = maybeInt {}
            ~~~~~~~~ ^ ~~~~~ 
                   //`- error: cannot force downcast in a type-casting pattern
                   // - fix-it: remove '!'
if case let intValue is Int = maybeInt {}
            ~~~~~~~~ ^ ~~~~
                   //`- error: use 'as' keyword to bind a matched value
                   // - fix-it: replace 'is' with 'as'

Resolves https://github.com/swiftlang/swift/issues/44631.

[jameesbrown]

@xedin Thanks for the review! Will address your comments shortly.

[xedin]

Please re-request a review once you are done to help me keep track of this.

[xedin]

@swift-ci please test

[jameesbrown]

@xedin Thanks for your help with this! It seems like the test failures should be fixed by https://github.com/swiftlang/swift/pull/76679.

[xedin]

@swift-ci please test

[xedin]

@swift-ci please test Windows platform

[xedin]

https://github.com/swiftlang/swift-testing/pull/731 @swift-ci please test Windows platform

[xedin]

@swift-ci please test macOS platform