SR-5753: Don't warn about constraints made redundant by inferred constraints

[DougGregor]

By default, don't warn about generic constraints that have been made
redundant by inferred constraints, e.g.,

func f<T: Hashable>(set: Set<T>) { }

T: Hashable is inferred by the Set parameter, but enough users are
annoyed or confused by this behavior that we'll turn it off by
default. The new command-line parameter
-warn-redundant-inferred-constraints enables these warnings.

Resolves SR-5753.

[DougGregor]

@swift-ci please smoke test

[DougGregor]

@jrose-apple I bet you're not gonna like this one :)

[jrose-apple]

I like the change in default, dislike that there's a warning flag. Who's turning the flag on?

[NachoSoto]

Love it.

[DougGregor]

The standard library and some test cases are currently turning the flag on.

[jrose-apple]

I guess I meant we shouldn't bother using it in the stdlib either. It's convenient when we remove hack constraints, but the original motivation seems to apply just as much to the stdlib: sometimes code is clearer when you can state a constraint explicitly that isn't obvious from the types that use the generic parameter.