Make two cp-name-parser.y constants "const"

This changes "backslashable" and "represented" in cp-name-parser.y to
be const.  This lets the compiler make them read-only (though in my
build it seems that GCC inlines them, which seems even better).

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

	* cp-name-parser.y (backslashable, represented): Now const.