[SR-13653] SourceKit-LSP: show full type signature in autocompletion

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Issue Description:

Currently, when typing max you get

max(_ x: Comparable, _ y: Comparable) -> Comparable

as a suggested type signature, which may led the user into thinking that the function accepts existential protocol types instead of a unique type "T: Comparable" in both the arguments and result (see attached image, P1 and P2 are protocols, C1 is a class).

It may be more useful to show

max<T: Comparable>(_ x: T, _ y: T) -> T

as suggested type signature, although less readable.

We have the same issue when typing in the function body:

protocol SomeProtocol {}
class SomeClass {}

func foo<T: Collection>(_ x: T) {
    x  // suggested as "x: Collection" instead of "x: T"
}

func bar<T: SomeClass & SomeProtocol>(_ x: T) { 
    x // suggested as "x: SomeProtocol" instead of "x: T" 
}

Start by reproducing the Swift completion examples for max, foo, and bar in SourceKit-LSP, then trace the completion path that formats displayed type signatures. Compare the current existential-style output with the generic signatures described here; done means completions preserve the type parameter and its constraints in both function declarations and local values.