add foreign-vectorcall test

Author: RalfJung

tests/ui/extern/auxiliary/foreign-vectorcall.rs

@@ -0,0 +1,7 @@
+#![feature(abi_vectorcall)]
+
+#[no_mangle]
+#[inline(never)]
+pub extern "vectorcall" fn call_with_42(i: i32) {
+    assert_eq!(i, 42);
+}

tests/ui/extern/extern-vectorcall.rs

@@ -1,10 +1,19 @@
 //@ run-pass
+//@ aux-build:foreign-vectorcall.rs
 //@ revisions: x64 x32
 //@ [x64]only-x86_64
 //@ [x32]only-x86
 
 #![feature(abi_vectorcall)]
 
+extern crate foreign_vectorcall;
+
+// Import this as a foreign function, that's the code path we are interested in
+// (LLVM has to do some name mangling here).
+extern "vectorcall" {
+    fn call_with_42(i32);
+}
+
 trait A {
     extern "vectorcall" fn test1(i: i32);
 }
@@ -24,4 +33,5 @@ extern "vectorcall" fn test2(i: i32) {
 fn main() {
     <S as A>::test1(1);
     test2(2);
+    unsafe { call_with_42(42) };
 }