Produce a special diagnostic when users call a function with an argument of

incomplete type (or a pointer/reference to such).

The causes of this problem are different enough to justify a different "design"
for the diagnostic.  Most notably, it doesn't give an operand index:
it's usually pretty obvious which operand is the problem, it adds a lot of
clutter to mention it, and the fix is usually in a different part of the file
anyway.

This is yet another diagnostic that should really have an analogue in the
non-overloaded case --- which should be much easier to write because of
the weaker space constraints.

llvm-svn: 94303