also test fastcall

Author: RalfJung

tests/codegen/abi-win64-zst.rs

@@ -1,3 +1,5 @@
+//@ compile-flags: -Z merge-functions=disabled
+
 //@ revisions: linux
 //@[linux] compile-flags: --target x86_64-unknown-linux-gnu
 //@[linux] needs-llvm-components: x86
@@ -31,6 +33,12 @@ extern "sysv64" fn pass_zst_sysv64(_: ()) {}
 #[no_mangle]
 extern "vectorcall" fn pass_zst_vectorcall(_: ()) {}
 
+// windows-gnu: define void @pass_zst_fastcall()
+// windows-msvc: define void @pass_zst_fastcall()
+#[no_mangle]
+#[cfg(windows)] // "fastcall" is not valid on 64bit Linux
+extern "fastcall" fn pass_zst_fastcall(_: ()) {}
+
 // For `extern "C"` functions, ZST are *not* ignored on windows-gnu.
 // That's likely an accident on their side but the ABI is what it is.