[SE-0470] Include isolated conformance checks for default value expressions

Per SE-0411, we compute the isolation of a default value expression
based on what isolation it requires. Include isolated conformance
checks in this computation, rather than always emitting diagnostics,
so that the combination of isolated default values + isolated
conformances works as expected.

Fixes rdar://150691429.

lib/Sema/CodeSynthesis.cpp

@@ -1723,7 +1723,8 @@ bool swift::hasLetStoredPropertyWithInitialValue(NominalTypeDecl *nominal) {
   });
 }
 
-/// Determine whether a synth
+/// Determine whether a synthesized requirement for the given conformance
+/// should be explicitly marked as 'nonisolated'.
 static bool synthesizedRequirementIsNonIsolated(
     const NormalProtocolConformance *conformance) {
   // @preconcurrency suppresses this.