gdb: avoid accessing global C++ language implementation functions

The function c_printchar is called from two places; it provides the
implementation of language_defn::printchar and it is called from
dwarf2_compute_name.

It would be nice to rename c_printchar as language_defn::printchar and
so avoid the trampoline.

To achieve this, instead of calling c_printchar directly from the
DWARF code, I lookup the C++ language object and call the printchar
member function.

In a later commit I can then rename c_printchar.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* dwarf2/read.c (dwarf2_compute_name): Call methods on C++
	language object instead of calling global functions directly.