Parse / AST: Allow let / var as argument labels with a warning.

The diagnostic is now a warning and the new message alerts the user that
though it is valid to have let and var as argument label names,
they are interpreted as argument labels, not keywords.

lib/Basic/StringExtras.cpp

@@ -30,8 +30,6 @@ using namespace camel_case;
 
 bool swift::canBeArgumentLabel(StringRef identifier) {
   return llvm::StringSwitch<bool>(identifier)
-    .Case("var", false)
-    .Case("let", false)
     .Case("inout", false)
     .Case("$", false)
     .Default(true);