[ast] Rename ParamSpecifier::{Transferring,ImplicitlyCopyableConsuming}.

The reason why I am doing this is that I am going to be changing transferring to
not be a true ParamSpecifier. Instead, it is going to be a bit on Param that
changes the default ParamSpecifier used. That being said, I cannot use consuming
for this purpose since consuming today implies no implicit copy semantics, which
we do not want unless the user specifically asks for it by writing consuming.

lib/Serialization/Deserialization.cpp

@@ -2884,7 +2884,7 @@ getActualParamDeclSpecifier(serialization::ParamDeclSpecifier raw) {
   CASE(Consuming)
   CASE(LegacyShared)
   CASE(LegacyOwned)
-  CASE(Transferring)
+  CASE(ImplicitlyCopyableConsuming)
   }
 #undef CASE
   return llvm::None;