PATCH RFA: Move tree codes used only by C++ frontend into cp/*

[Gabriel Dos Reis]

Ian Lance Taylor <ian@airs.com> writes:

| There are several tree codes currently defined in the common C/C++
| code which are only used by C++.  These are the basic language
| constructs, such as FOR_STMT, WHILE_STMT, etc.  The C frontend now
| simply generates GENERIC for these cases.  The C++ frontend continues
| to use these language specific codes.

A meta question:  Have we agreed that it is a wrong idea to unify the
C and C++ front-ends, therefore either front-end is better developed
independently?

-- Gaby