Resolve CodingKeys before visiting member decls in the primary

Force this check to occur so that the interface types of members can successfully make reference to CodingKeys.  This is an egregious emergent behavior of Codable that currently forces two protocol conformance checks just to synthesize a type witness.  We can definitely get away with just creating the witness up front.

rdar://56844567

Author: CodaFi

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -333,6 +333,12 @@ static void checkInheritanceClause(
   }
 }
 
+static void installCodingKeysIfNecessary(NominalTypeDecl *NTD) {
+  auto req =
+    ResolveImplicitMemberRequest{NTD, ImplicitMemberAction::ResolveCodingKeys};
+  (void)evaluateOrDefault(NTD->getASTContext().evaluator, req, false);
+}
+
 // Check for static properties that produce empty option sets
 // using a rawValue initializer with a value of '0'
 static void checkForEmptyOptionSet(const VarDecl *VD) {
@@ -1666,6 +1672,8 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
 
     TypeChecker::addImplicitConstructors(SD);
 
+    installCodingKeysIfNecessary(SD);
+
     for (Decl *Member : SD->getMembers())
       visit(Member);