Put More Faith In Raw Values From Synthesized Enums

By requestifying the raw value accessor and pushing the semantic check
into IRGen, we are now partially validating enums that are
synthesized by the clang importer and deserialization - something that
we were not doing before.  The previous check would only skip
the auto-incrementer step when this was the case.  We now also skip the
uniqueness check.

Resolves rdar://56037318

Author: CodaFi

lib/ClangImporter/ImportDecl.cpp

@@ -2853,6 +2853,7 @@ namespace {
         auto enumDecl = Impl.createDeclWithClangNode<EnumDecl>(
             decl, AccessLevel::Public, loc, enumName,
             Impl.importSourceLoc(decl->getLocation()), None, nullptr, enumDC);
+        enumDecl->setHasFixedRawValues();
         enumDecl->computeType();
 
         // Annotate as 'frozen' if appropriate.

lib/Sema/TypeCheckDecl.cpp

@@ -1654,6 +1654,13 @@ EnumRawValuesRequest::evaluate(Evaluator &eval, EnumDecl *ED,
       continue;
     }
 
+    // If the raw values of the enum case are fixed, then we trust our callers
+    // to have set things up correctly.  This comes up with imported enums
+    // and deserialized @objc enums which always have their raw values setup
+    // beforehand.
+    if (ED->LazySemanticInfo.hasFixedRawValues())
+      continue;
+
     // Check that the raw value is unique.
     RawValueKey key{prevValue};
     RawValueSource source{elt, lastExplicitValueElt};