Implement function calls for Ada

Implement function calls for Ada

This implements function calls for Ada.  This takes a different
approach than that used for other languages, primarily because Ada
requires special treatment generally.  The "ordinary" special case for
just the callee didn't really apply neatly here; there's only one case
in Ada needing special callee treatment.

gdb/ChangeLog
2021-03-08  Tom Tromey  <tom@tromey.com>

	* ada-lang.c (ada_funcall_operation::evaluate): New method.
	* ada-exp.h (class ada_var_msym_value_operation) <get_symbol>: New
	method.
	(class ada_funcall_operation): New.