Ensure that debugger calls to signature-less functions default to
passing arguments in the fixed style.

We have an abstraction for deciding this, but it's (1) deep in
IR-generation, (2) necessarily tied to exact argument lists, and
(3) triggered by unprototyped function types, which we can't
legitimately make in C++ mode.  So this solution, wherein Sema
rewrites the function type to an exact prototype but leaves the
variadic bit enabled so as to request x86-64-like platforms to
pass the extra variadic info, is very much a hack, but it's one
that works in practice on the platforms that LLDB will support
in the medium term --- the only place we know of where it's a
problem is instance methods in Windows, where variadic functions
are implicitly cdecl.  We may have a more abstracted base on which
to build a solution by then.

rdar://13731520