It's important to note that no LLVM code produced by the C or C++ frontends can
directly have this. It can only be triggered in two ways:
LLVM code written by hand (like the testcase)
Optimizations which build new functions or remove arguments could cause this.
W.R.T case #2, the dead argument elimination pass (which is the only one which
could trigger the bug) now has an explicit hack to work around this (it does not
delete all of the fixed arguments to a varargs function).
Because of this, I'm reducing the severity of the bug to "minor".
I'm reducing this to 'enhancement', because there is no way to implement this
with ANSI C. Perhaps there are some extensions to some compiler that could be
used to implement this, but I don't think GCC has them.