Rollup merge of #47618 - mrhota:dw_at_noreturn, r=michaelwoerister

Teach rustc about DW_AT_noreturn and a few more DIFlags

We achieve two small things with this PR:
1. We provide definitions for a few additional llvm debuginfo flags
1. We _use_ one of these new flags, `FlagNoReturn`, and add it to debuginfo for functions with the never return type (`!`).

src/test/codegen/noreturnflag.rs

@@ -0,0 +1,24 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// ignore-tidy-linelength
+// min-llvm-version 4.0
+
+// compile-flags: -g -C no-prepopulate-passes
+
+// CHECK: {{.*}}DISubprogram{{.*}}name: "foo"{{.*}}DIFlagNoReturn
+
+fn foo() -> ! {
+    loop {}
+}
+
+pub fn main() {
+    foo();
+}