ANSI C requires that a call to an unprototyped function type succeed

ANSI C requires that a call to an unprototyped function type succeed
if the definition has a non-variadic prototype with compatible
parameters.  Therefore, the default rule for such calls must be to
use a non-variadic convention.  Achieve this by casting the callee to
the function type with which it is required to be compatible, unless
the target specifically opts out and insists that unprototyped calls
should use the variadic rules.  The only case of that I'm aware of is
the x86-64 convention, which passes arguments the same way in both
cases but also sets a small amount of extra information;  here we seek
to maintain compatibility with GCC, which does set this when calling
an unprototyped function.

Addresses PR10810 and PR10713.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@140241 91177308-0d34-0410-b5e6-96231b3b80d8