[CS] Fix a crash in AllowArgumentMismatch

[theblixguy]

The compiler was crashing because AllowArgumentMismatch relies on getFunctionArgApplyInfo() returning a non-optional FunctionArgApplyInfo value, which we failed to provide in a situation when simplifyLocator couldn't simplify a locator where a TypeExpr was provided as a subscript argument.

Resolves SR-12990
Resolves rdar://problem/64303153

[xedin]

I think we have it setup this way on purpose, AllowArgumentMismatch should only be attached to the argument-to-parameter conversion failures and getFunctionArgApplyInfo should be able to extract information based on that locator. Why does it fail in this case?

[theblixguy]

@xedin Oh sorry, I didn't realise it was intentional. In that case, I think it might be better to fix simplifyLocator because we have a locator like apply argument -> comparing call argument #0 to parameter #0 and when we simplify ApplyArgument we get a TypeExpr anchor and when we try to simplify ApplyArgToParam we don't handle a TypeExpr and so simplifyLocatorToAnchor returns nullptr (due to path being non-empty).

[xedin]

@theblixguy Right, that's what I expected, I think we need to adjust logic in simplifyLocatorToAnchor to handle cases we overlooked...

[theblixguy]

@xedin Sounds good, that's what I have done now!

[xedin]

Thank you!

[xedin]

@swift-ci please smoke test

[xedin]

@theblixguy Could you open a PR for 5.3? You'd have to adjust isExpr to isa I think.

[theblixguy]

Sure, here's the 5.3 PR: #32361