ASTGen: Suppress spurious retroactive conformance warnings.

The Swift 6.2 compiler has a bug where it diagnoses inherited conformances as
retroactive even if they are inherited through a protocol that has been written
module-qualified in the inheritance list. Work around this bug to avoid
superflous warnings.

NFC.

Author: tshortli

lib/ASTGen/Sources/ASTGen/Bridge.swift

@@ -31,21 +31,21 @@ extension BridgedNullable {
   }
 }
 
-extension SourceLoc: /*@retroactive*/ swiftASTGen.BridgedNullable {}
-extension Identifier: /*@retroactive*/ swiftASTGen.BridgedNullable {}
-extension BridgedNullableDecl: /*@retroactive*/ swiftASTGen.BridgedNullable {}
-extension BridgedNullableExpr: /*@retroactive*/ swiftASTGen.BridgedNullable {}
-extension BridgedNullableStmt: /*@retroactive*/ swiftASTGen.BridgedNullable {}
-extension BridgedNullableTypeRepr: /*@retroactive*/ swiftASTGen.BridgedNullable {}
-extension BridgedNullablePattern: /*@retroactive*/ swiftASTGen.BridgedNullable {}
-extension BridgedNullableGenericParamList: /*@retroactive*/ swiftASTGen.BridgedNullable {}
-extension BridgedNullableTrailingWhereClause: /*@retroactive*/ swiftASTGen.BridgedNullable {}
-extension BridgedNullableParameterList: /*@retroactive*/ swiftASTGen.BridgedNullable {}
-extension BridgedNullablePatternBindingInitializer: /*@retroactive*/ swiftASTGen.BridgedNullable {}
-extension BridgedNullableDefaultArgumentInitializer: /*@retroactive*/ swiftASTGen.BridgedNullable {}
-extension BridgedNullableCustomAttributeInitializer: /*@retroactive*/ swiftASTGen.BridgedNullable {}
-extension BridgedNullableArgumentList: /*@retroactive*/ swiftASTGen.BridgedNullable {}
-extension BridgedNullableVarDecl: /*@retroactive*/ swiftASTGen.BridgedNullable {}
+extension SourceLoc: /*@retroactive*/ swiftASTGen.BridgedNullable, Swift.ExpressibleByNilLiteral {}
+extension Identifier: /*@retroactive*/ swiftASTGen.BridgedNullable, Swift.ExpressibleByNilLiteral {}
+extension BridgedNullableDecl: /*@retroactive*/ swiftASTGen.BridgedNullable, Swift.ExpressibleByNilLiteral {}
+extension BridgedNullableExpr: /*@retroactive*/ swiftASTGen.BridgedNullable, Swift.ExpressibleByNilLiteral {}
+extension BridgedNullableStmt: /*@retroactive*/ swiftASTGen.BridgedNullable, Swift.ExpressibleByNilLiteral {}
+extension BridgedNullableTypeRepr: /*@retroactive*/ swiftASTGen.BridgedNullable, Swift.ExpressibleByNilLiteral {}
+extension BridgedNullablePattern: /*@retroactive*/ swiftASTGen.BridgedNullable, Swift.ExpressibleByNilLiteral {}
+extension BridgedNullableGenericParamList: /*@retroactive*/ swiftASTGen.BridgedNullable, Swift.ExpressibleByNilLiteral {}
+extension BridgedNullableTrailingWhereClause: /*@retroactive*/ swiftASTGen.BridgedNullable, Swift.ExpressibleByNilLiteral {}
+extension BridgedNullableParameterList: /*@retroactive*/ swiftASTGen.BridgedNullable, Swift.ExpressibleByNilLiteral {}
+extension BridgedNullablePatternBindingInitializer: /*@retroactive*/ swiftASTGen.BridgedNullable, Swift.ExpressibleByNilLiteral {}
+extension BridgedNullableDefaultArgumentInitializer: /*@retroactive*/ swiftASTGen.BridgedNullable, Swift.ExpressibleByNilLiteral {}
+extension BridgedNullableCustomAttributeInitializer: /*@retroactive*/ swiftASTGen.BridgedNullable, Swift.ExpressibleByNilLiteral {}
+extension BridgedNullableArgumentList: /*@retroactive*/ swiftASTGen.BridgedNullable, Swift.ExpressibleByNilLiteral {}
+extension BridgedNullableVarDecl: /*@retroactive*/ swiftASTGen.BridgedNullable, Swift.ExpressibleByNilLiteral {}
 
 extension Identifier: /*@retroactive*/ Swift.Equatable {
   public static func == (lhs: Self, rhs: Self) -> Bool {