[Do Not Merge] Enable one-way closure parameters by default. #32222
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Introduce an experimental mode (behind the flag
`experimental-one-way-closure-params`) that places one-way
constraints between closure parameter types and references to those
parameters within the body of the closure. The intent here is to
break up constraint systems further, potentially improving type
checking performance and making way for larger closure bodies to be
supported.
This is a source-breaking change when the body of a single-expression
closure is used to determine the parameter types. One obvious example
is when there is no contextual type, e.g.,
let _ = { $0 + 1 }
this type-checks today because `1` becomes `Int`, which matches the
`+` overload with the type `(Int, Int) -> Int`, determining the
parameter type `Int` for the closure. Such code would not type-check
with one-way constraints.
There are two places in our normal build that are missing type annotations that would become required with one-way closure parameters. Introduce type annotations so that we can build enough of the code base to perform additional testing. These changes are unnecessary (but harmless) if nothing comes of the one-way closure experiment.
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Experiment with enabling one-way closure parameters by default.