Describe the issue

MSYS2 Clang miscompiles va_start.

Steps to Reproduce the Problem

1. Compile this file with Clang in MSYS2. Optimizations neither break nor fix the issue.

#include <stdarg.h>
#include <stdio.h>

void my_printf(const char *fmt, ...)
{
    va_list ap;
    va_start(ap, fmt);
    vprintf(fmt, ap);
    va_end(ap);
}

int main(void)
{
    my_printf("%s\n", "Hello, world!");
}

2. Run it.
3. Garbage is outputted to the console.
4. Note that GCC compiles it fine, so it is not a libc problem.

Additional Context: Operating System, Screenshots

$ clang -v
clang version 11.0.0 (https://github.com/msys2/MSYS2-packages a5a028a0811f03c8f9697bf80c2e28111628ffff)
Target: x86_64-pc-windows-msys
Thread model: posix
InstalledDir: /usr/bin

Also reported here at LLVM: https://bugs.llvm.org/show_bug.cgi?id=48624