[Diagnostics] Offer 'is' replacement for unused 'if let' expression with optional operand

[mininny]

let foo: Any? = 42 as Int

// current fix-it: Replace 'let bar = foo as? Int' with '(foo as? Int) != nil'
// better fix-it: Replace 'let bar = foo as? Int' with 'foo is Int'

if let bar = foo as? Int { 
    print("'foo' is of type 'Int'")
}

Currently, the above code does not produce the better fix-it for replacing the expression with is because the expression was wrapped inside OptionalEvaluationExpr.

This PR enables optional typed values to produce is replacement fixit.

Resolves SR-14646

[xedin]

Thank you for the contribution! Code makes sense to me, I have left a couple of minor comments inline.

One more thing - please use clang-format to format the code, you can do that via git-clang-format as well after git adding the files, that tool could be found in clang/tools/clang-format at your llvm-project checkout.

[xedin]

There might be parens involved so you'd need to use getSubExpr->getSemanticsProvidingExpr() here otherwise it would fallback to != nil diagnostics again in case like (bb) as? int. Also please add that to https://github.com/apple/swift/pull/37897/files#diff-cb1dfa36aab299d0abafe04242dbebf8a555ad1a20041b47c1299f4687935f41R2885.

[mininny]

Thanks for the comment :) I modified the code to use getValueProvidingExprto cover try expressions as well, and added necessary tests!

[mininny]

One more thing - please use clang-format to format the code, you can do that via git-clang-format as well after git adding the files, that tool could be found in clang/tools/clang-format at your llvm-project checkout.

Thanks! I applied clang-format to format the code :)

[xedin]

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[LucianoPAlmeida]

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