Correct behavior of fastcall when default CC is set.

Fastcall doesn't support variadic function calls, so
setting the default calling convention to Fastcall would
result in incorrect code being emitted for these conditions.

This patch adds a 'variadic' test to the default calling conv
test, as well as fixes the behavior of fastcall.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@316528 91177308-0d34-0410-b5e6-96231b3b80d8