gdb: fix regression in evaluate_funcall for non C++ like cases

This regression, as it is exposed by the test added in this commit,
first became noticable with this commit:

  commit d182f2797922a305fbd1ef6a483cc39a56b43e02
  Date:   Mon Mar 8 07:27:57 2021 -0700

      Convert c-exp.y to use operations

But, this commit only added converted the C expression parser to make
use of code that was added in this commit:

  commit a00b7254fb614af557de7ae7cc0eb39a0ce0e408
  Date:   Mon Mar 8 07:27:57 2021 -0700

      Implement function call operations

And it was this second commit that actually introduced the bugs (there
are two).

In structop_base_operation::evaluate_funcall we build up an argument
list in the vector vals.  Later in this function the argument list
might be passed to value_struct_elt.

Prior to commit a00b7254fb614 the vals vector (or argvec as it used to
be called) stored the value for the function callee in the argvec at
index 0.  This 'callee' value is what ends up being passed to
evaluate_subexp_do_call, and represents the function to be called, the
value contents are the address of the function, and the value type is
the function signature.  The remaining items held in the argvec were
the values to pass to the function.  For a non-static member function
the `this' pointer would be at index 1 in the array.

After commit a00b7254fb614 this callee value is now held in a separate
variable, not the vals array.  So, for non-static member functions,
the `this' pointer is now at index 0, with any other arguments after
that.

What this means is that previous, when we called value_struct_elt we
would pass the address of argvec[1] as this was the first argument.
But now we should be passing the address of vals[0].  Unfortunately,
we are still passing vals[1], effectively skipping the first
argument.

The second issue is that, prior to commit a00b7254fb614, the argvec
array was NULL terminated.  This is required as value_struct_elt
calls search_struct_method, which calls typecmp, and typecmp requires
that the array have a NULL at the end.

After commit a00b7254fb614 this NULL has been lost, and we are
therefore violating the API requirements of typecmp.

This commit fixes both of these regressions.  I also extended the
header comments on search_struct_method and value_struct_elt to make
it clearer that the array required a NULL marker at the end.

You will notice in the test attached to this commit that I test
calling a non-static member function, but not calling a static member
function.  The reason for this is that calling static member functions
is currently broken due to a different bug.  That will be fixed in a
later patch in this series, at which time I'll add a test for calling
a static member function.

gdb/ChangeLog:

	PR gdb/27994
	* eval.c (structop_base_operation::evaluate_funcall): Add a
	nullptr to the end of the args array, which should not be included
	in the argument array_view.  Pass all the arguments through to
	value_struct_elt.
	* valops.c (search_struct_method): Update header comment.
	(value_struct_elt): Likewise.

gdb/testsuite/ChangeLog:

	PR gdb/27994
	* gdb.cp/method-call-in-c.cc: New file.
	* gdb.cp/method-call-in-c.exp: New file.

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