`_const` parameter modifier (and other underscored parameter modifiers)

[hooman (Hooman Mehr)]

It is this:

[Returned for revision] SE-0359: Build-Time Constant Values

Hello Swift community, The review for SE-0359 "Build-Time Constant Values" ended on June 6th. The proposal is being returned for revision. The review brought up a number of concerns about the proposal, but the overarching theme is that it isn't clear how this proposal fits into the overall picture for build-time constant evaluation in Swift, and with that it does not stand well on its own. This manifests in a few different ways: Concerns about the virality of @const: Noted in several places…