Add resolvedOrVirtualFunction, fix issue #6542

[mppf]

This PR takes two steps to fix issue #6542:

1. It introduces a function, CallExpr::resolvedOrVirtualFunction, factoring out a code pattern used already in callDestructors and errorHandling. The pattern is to find the resolved function, or, for a PRIM_VIRTUAL_METHOD_CALL, find one of the possible virtual methods that could be called at runtime. This function is appropriate to call for code that identifies calls to transform based upon the signature of the called function.
2. It calls this function in two places in order to resolve issue Returning by-ref values from overridden methods and not capturing them at call site #6542.

What was going wrong with issue #6542? The compiler includes a transformation that happens during resolution, insertReturnTemps, which finds all calls to functions that return a value that don't use the result of that return. In that case it adds a temporary variable to capture the returned value.

Code in callDestructors / ReturnByRef relied on calls to transformable functions having a variable to capture the returned value.

In the case of virtual method calls, insertReturnTemps was not transforming the call, and so the code pattern in issue #6542 led to failed assertions in callDestructors.

The important part of this fix is to adjust insertReturnTemps to handle virtual calls (PRIM_VIRTUAL_METHOD_CALL).

Passed full local testing.
Reviewed by @noakesmichael - thanks!

[mppf]

@noakesmichael - would you mind looking at this bug fix? Thanks.