[flang] Prevent result symbol to be converted into function when they are called.

+ Fix issue 589.
+ Catch empty subscript list in array reference
In name resolution, when skimming through the execution statement of a
function, calls to the result symbol should not trigger the conversion
of this symbol to a function symbol. The result is a data object and
cannot be called unless it was explictly declared to be a procedure
pointer.
Notably, recursive function calls cannot be made if RESULT was not used.

The symbol is prevented from being transformed into a function
symbol by transforming it into an object before skimming through the
executable statement. This is done after processing all the
specifications so that if the result actually is a procedure pointer,
the call to `ConvertToObjectEntity` introduced by this commit will
not convert it to an object by mistake.

This commit also introduce a check when fixing misparsed function
reference into array reference to verify the array reference has
array subscripts. Currently this went uncaught. It is not possible
to complain later in expressions because the subscript list of
expression might be empty for unrelated error recovery reasons
(e.g. if an entity of the wrong type appeared as susbcript).

Add related tests.

Original-commit: flang-compiler/f18@2fd8b65f58aadc567dd23d64e4654304a42f6176
Reviewed-on: https://github.com/flang-compiler/f18/pull/631
Tree-same-pre-rewrite: false