[5.3] Check SPI usage of implementation-only types, in frozen types and on protocol requirements

[xymus]

Cherry-pick of #31744 and #31737.

• Explanation:
  Check for three invalid uses of Swift SPI:
  • Exposing implementation-only types in SPI function signatures could make the compiler emit invalid .private.swiftinterface files.
  • Marking a stored property in a frozen struct as SPI would prevent external clients from knowing the correct struct layout.
  • Declaring a protocol requirement as SPI in a public protocol could break external clients unless there is a default implementation in the defining module.
• Scope: The checks apply only to the use of the @_spi attribute.
• Risk: Low
• Testing: Added new tests for each case.
• Resolves rdar://63193289
• Reviewed by @nkcsgexi and @brentdax

[xymus]

@swift-ci Please test

[xymus]

@swift-ci Please nominate