[metadata prespecialization] Check conformances.

[nate-chandler]

Previously, the metadata accessor for a generic type for which some metadata prespecialization was done only tested that the type metadata arguments were equal to those of the prespecialization. If the generic type had an argument which was constrained to conform to a protocol, the particular conformance to that protocol was determined at compile time, but the conformance was ignored in the metadata accessor. As a result it was possible--in certain multi-module cases--for the metadata accessor to incorrectly return a prespecialized metadata record whose type arguments matched the type arguments passed to the accessor but whose conformance arguments did not.

For example, given the following,

Base:
    struct K {}
    protocol P {}

Conformance1:
    import Base
    struct G<T : P> {}
    extension K : P {} // first conformance
    prespecialize( G<K>.self )

Conformance2:
    import Base
    extension K : P {} // second conformance

the metadata accessor for G defined in Conformance1 would behave like

MetadataResponse `metadata accessor for G`(
    MetadataRequest request,
    const Metadata *M,
    const WitnessTable *WT) {
  if (M == `type metadata for K`) {
    return `canonical prespecialized type metadata for G<K>`
  }
  return swift_getGenericMetadata(request, {M, WT});
}

Here, the WitnessTable argument is the witness table describing a conformance of the type whose metadata is provided to the protocol P.

The incorrect behavior occurs when calling the metadata accessor with these arguments:

some request
type metadata for K
protocol witness table for Base.K : Base.P in Conformance2

The accessor would return the canonical prespecialized type metadata for G<K>. The problem is that the prespecialized metadata contains the following generic arguments:

type metadata for K
protocol witness table for Base.K : Base.P in Conformance1;

Specificallly, the witness table is for the conformance from Conformance1, not the conformance from Conformance2.

Here, the problem is addressed by testing that the witness tables passed into the accessor are for the same conformance as the witness table referred to by the prespecialized record. Now, the metadata accessor for G will behave like

MetadataResponse `metadata accessor for G`(
    MetadataRequest request,
    const Metadata *M,
    const WitnessTable *WT) {
  if (M == `type metadata for K`
      swift_compareProtocolConformanceDescriptors(
        WT->getDescription(),
        `protocol conformance descriptor for Base.K : Base.P in Conformance1`)
     ) {
    return `canonical prespecialized type metadata for G<K>`
  }
  return swift_getGenericMetadata(request, {M, WT});
}

Consequently, when the accessor is called with the same arguments as before, the call to swift_compareProtocolConformanceDescriptors will return false and the non-matching prespecialized metadata will not be returned.

Dependent on #32471 .

[nate-chandler]

@swift-ci please test

[swift-ci]

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[nate-chandler]

@swift-ci please clean test os x platform

[nate-chandler]

@swift-ci please test

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[nate-chandler]

@swift-ci please clean test macos platform

[nate-chandler]

@swift-ci please test and merge

[nate-chandler]

@swift-ci please test and merge

[nate-chandler]

@swift-ci please test

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Build failed
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[swift-ci]

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@swift-ci please test macos platform

[nate-chandler]

@swift-ci please test macos platform

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Build failed
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[nate-chandler]

@swift-ci please clean test macos platform