Add support for diagnosing calls to closures that return a generic
non-Escapable result.
Closures do not yet model lifetime dependencies. The diagnostics have
a special case for handling nonescaple result with no lifetime
dependence, but it previously only handled direct results. This fix handles
cases like the following:
func callIndirectClosure<T>(f: () -> NE<T>) -> NE<T> {
f()
}
Fixes rdar://134318846 ([nonescapable] diagnose function types with nonescapable results)
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