[Isolated conformances] Start parsing 'nonisolated' on conformances

It doesn't mean anything yet, but parse it and make sure it reproduces
in the module interface.

Author: DougGregor

lib/Parse/ParseType.cpp

@@ -413,6 +413,15 @@ ParserResult<TypeRepr> Parser::parseTypeScalar(
     return makeParserCodeCompletionResult<TypeRepr>(ET);
   }
 
+  // "nonisolated" for attribute lists.
+  if (reason == ParseTypeReason::InheritanceClause &&
+      Tok.isContextualKeyword("nonisolated")) {
+    SourceLoc nonisolatedLoc = consumeToken();
+    parsedAttributeList.Attributes.push_back(
+        TypeAttribute::createSimple(Context, TypeAttrKind::Nonisolated,
+                                    SourceLoc(), nonisolatedLoc));
+  }
+
   // Parse generic parameters in SIL mode.
   GenericParamList *generics = nullptr;
   SourceLoc substitutedLoc;

lib/Sema/TypeCheckType.cpp

@@ -3504,7 +3504,8 @@ TypeResolver::resolveAttributedType(TypeRepr *repr, TypeResolutionOptions option
 
   if (handleInheritedOnly(claim<UncheckedTypeAttr>(attrs)) ||
       handleInheritedOnly(claim<PreconcurrencyTypeAttr>(attrs)) ||
-      handleInheritedOnly(claim<UnsafeTypeAttr>(attrs)))
+      handleInheritedOnly(claim<UnsafeTypeAttr>(attrs)) ||
+      handleInheritedOnly(claim<NonisolatedTypeAttr>(attrs)))
     return ty;
 
   if (auto retroactiveAttr = claim<RetroactiveTypeAttr>(attrs)) {

test/ModuleInterface/isolated_conformance.swift

@@ -14,3 +14,9 @@ public class MyClass { }
 extension MyClass: @MainActor MyProtocol {
   @MainActor public func f() { }
 }
+
+extension MyClass: nonisolated Equatable {
+  nonisolated public static func ==(lhs: MyClass, rhs: MyClass) -> Bool {
+    false
+  }
+}