Implement some agent expressions for Ada

Implement some agent expressions for Ada

Ada historically has not implemented agent expressions, and some Ada
constructs probably cannot reasonably be converted to agent
expressions.  However, a subset of simple operations can be, and this
patch represents a first step in that direction.

On one internal AdaCore test case, this improves the performance of a
conditional breakpoint from 5 minutes to 5 seconds.

The main tricky part in this patch is ensuring the converted
expressions detect the cases that will not work.  This is done by
examining the code in the corresponding evaluation methods.