[Observation] Create nonisolated conformances to Observable

This is needed to make default-main-actor mode work well with
Observable. Fixes rdar://150419628.

Author: DougGregor

lib/Macros/Sources/ObservationMacros/ObservableMacro.swift

@@ -381,7 +381,7 @@ extension ObservableMacro: ExtensionMacro {
     }
 
     let decl: DeclSyntax = """
-        extension \(raw: type.trimmedDescription): \(raw: qualifiedConformanceName) {}
+        extension \(raw: type.trimmedDescription): nonisolated \(raw: qualifiedConformanceName) {}
         """
     let ext = decl.cast(ExtensionDeclSyntax.self)
 

lib/Sema/TypeCheckMacros.cpp

@@ -2152,8 +2152,8 @@ std::optional<unsigned> swift::expandExtensions(CustomAttr *attr,
     for (auto i : inheritedTypes.getIndices()) {
       auto constraint =
           TypeResolution::forInterface(
-              extension->getDeclContext(),
-              TypeResolverContext::GenericRequirement,
+              extension,
+              TypeResolverContext::Inherited,
               /*unboundTyOpener*/ nullptr,
               /*placeholderHandler*/ nullptr,
               /*packElementOpener*/ nullptr)