Use 'const' in ada-exp.y

I found a few spots in ada-exp.y that could use 'const'.
Tested by rebuilding.

2021-07-02  Tom Tromey  <tromey@adacore.com>

	* ada-exp.y (chop_selector, chop_separator, write_selectors)
	(write_ambiguous_var, get_symbol_field_type): Use const.