[SR-14824] Improve diagnostic for multi-statement closures instead of saying "too complex closure return type"

[typesanitizer]

Previous ID	SR-14824
Radar	rdar://problem/79742920
Original Reporter	@typesanitizer
Type	Improvement
Status	Closed
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement, DiagnosticsQoI, StarterBug, TypeChecker
Assignee	jiaren wang (JIRA)
Priority	Medium

md5: 36dfe752f68258a70ceeb284792464d3

Issue Description:

See https://forums.swift.org/t/too-complex-closure-return-type/49897 – it's not clear why a closure is "too complex" because from a user perspective, it looks quite simple.

In case there are multiple statements, we can provide a more specialized diagnostic saying that the compiler doesn't support inferring types of closures with multiple statements.

_ = { // error: Unable to infer complex closure return type; add explicit type to disambiguate
    _ = "" // or anything else here
    return ""
} 

[typesanitizer]

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