Introduce class adl_func_operation

Introduce class adl_func_operation

This adds class adl_func_operation, which implements
argument-dependent lookup function calls.

Other function calls will be handled in a different way.  However,
because ADL calls were created in a single spot in the C++ parser, and
because they had different semantics from the other cases, it was
convenient to treat them specially.

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

	* expop.h (class adl_func_operation): New.
	* eval.c (adl_func_operation::evaluate): New method.