gdb/objc: make objc_demangle a member function of objc_language

gdb/objc: make objc_demangle a member function of objc_language

Makes the objc_demangle helper function a member function of
objc_language (by renaming it to be the demangle_symbol member
function).

I also fixed some of the obvious coding standard violations in
obj_demangle, so the '&&' operators are now at the start of the line,
not the end.  Comparison to nullptr are now made explicit, as are
comparisons to the null character.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* objc-lang.c (objc_demangle): Renamed to
	objc_language::demangle_symbol, and moved later in the file.
	(objc_language::sniff_from_mangled_name): Call demangle_symbol
	member function.
	(objc_language::demangle_symbol): Defined outside of class
	declaration.  The definition is the old objc_demangle with NULL
	changed to nullptr, and if conditions relating to nullptr pointers
	or null character checks made explicit.
	* objc-lang.h (objc_demangle): Delete declaration.