Use const more in c-lang.c

This changes a few functions in c-lang.c to add "const" to parameters
and return types.  Tested by rebuilding.

gdb/ChangeLog
2020-11-13  Tom Tromey  <tom@tromey.com>

	* c-lang.c (convert_ucn, convert_octal, convert_hex)
	(convert_escape, parse_one_string): Constify.