Delete more cpplib / C front end duplication

[Neil Booth]

Neil Booth wrote:-

> 	* c-lex.c (read_escape, read_ucs): Delete.
> 	(lex_string): Use cpp_parse_escape.
> 	* cpplex.c (read_ucs): Rename maybe_read_ucs.  Do nothing
> 	if not C++ or not C99.  Warn if traditional.
> 	(parse_escape): Rename cpp_parse_escape.  Make the bitmask
> 	unsigned.  Warn for out-of-range escape sequences here.
> 	Update to use maybe_read_ucs.
> 	(cpp_interpret_charconst):  Don't warn here.
> 	* cpplib.h (cpp_parse_escape): New exported function.

I'm going to apply this.  It has no regressions, Joseph has no issues
with it, and it's basically CPP land anyway.

Neil.