[SE-0328] Enable structural opaque result types.

[hborla]

• Enable structural opaque types by default
• Remove the -enable-experimental-structural-opaque-types frontend flag
• Ban opaque types in parameter position, per the acceptance decision of SE-0328. Note that this restriction applies to some appearing anywhere in the parameter type, including positions that do not consume values from the caller, e.g. func test() -> ((some P) -> Void) -> Void { ... }

Resolves: rdar://85275081

[hborla]

@swift-ci please smoke test

[hborla]

@swift-ci please smoke test Linux platform

[hborla]

@swift-ci please test Windows platform

[hborla]

@swift-ci please test Windows platform

[hborla]

Failed Tests (1):
  Swift(windows-x86_64) :: Serialization/path_obfuscator.swift

Windows test failure will be fixed by this revert: #40368

[hborla]

@swift-ci please test Windows platform