Remove a static buffer from cp-name-parser.y

This removes a static buffer from cp-name-parser.y by replacing the
fixed-sized buffer with a std::string out parameter.

gdb/ChangeLog
2018-06-01  Tom Tromey  <tom@tromey.com>

	* python/py-type.c (typy_legacy_template_argument): Update.
	* cp-support.h (cp_demangled_name_to_comp): Update.
	* cp-name-parser.y (cp_demangled_name_to_comp): Change errmsg
	parameter to be a "std::string *".
	(main): Update.