[Type checker diags] Check generic arguments for any generic function.

This includes functions without type parameters that are generic
because they are members of a generic context. This addresses a
diagnostics regression because the '==' operators for Optional, Array,
and Dictionary were made into members.

The resulting error message is a little ugly, but is more informative
(and much shorter) than what we had before.

Author: DougGregor

lib/Sema/CSDiag.cpp

@@ -4844,7 +4844,7 @@ bool FailureDiagnosis::diagnoseArgumentGenericRequirements(
     AFD = dyn_cast<AbstractFunctionDecl>(candidate);
   }
 
-  if (!AFD || !AFD->isGeneric() || !AFD->hasInterfaceType())
+  if (!AFD || !AFD->getGenericSignature() || !AFD->hasInterfaceType())
     return false;
 
   auto env = AFD->getGenericEnvironment();