Never return uninhabited values at all

Functions with uninhabited return values are already marked `noreturn`,
but we were still generating return instructions for this. When running
with `-C passes=lint`, LLVM prints:

    Unusual: Return statement in function with noreturn attribute

The LLVM manual makes a stronger statement about `noreturn` though:

> This produces undefined behavior at runtime if the function ever does
dynamically return.

We now emit an `abort` anywhere that would have tried to return an
uninhabited value.

src/librustc_codegen_ssa/mir/block.rs

@@ -238,6 +238,13 @@ impl<'a, 'tcx: 'a, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 }
             }
         }
+        if self.fn_ty.ret.layout.abi.is_uninhabited() {
+            // Functions with uninhabited return values are marked `noreturn`,
+            // so we should make sure that we never actually do.
+            bx.abort();
+            bx.unreachable();
+            return;
+        }
         let llval = match self.fn_ty.ret.mode {
             PassMode::Ignore(IgnoreMode::Zst) | PassMode::Indirect(..) => {
                 bx.ret_void();

src/test/codegen/noreturn-uninhabited.rs

@@ -0,0 +1,32 @@
+// compile-flags: -g -C no-prepopulate-passes
+// ignore-tidy-linelength
+
+#![crate_type = "lib"]
+
+#[derive(Clone, Copy)]
+pub enum EmptyEnum {}
+
+#[no_mangle]
+pub fn empty(x: &EmptyEnum) -> EmptyEnum {
+    // CHECK: @empty({{.*}}) unnamed_addr #0
+    // CHECK-NOT: ret void
+    // CHECK: call void @llvm.trap()
+    // CHECK: unreachable
+    *x
+}
+
+pub struct Foo(String, EmptyEnum);
+
+#[no_mangle]
+pub fn foo(x: String, y: &EmptyEnum) -> Foo {
+    // CHECK: @foo({{.*}}) unnamed_addr #0
+    // CHECK-NOT: ret %Foo
+    // CHECK: call void @llvm.trap()
+    // CHECK: unreachable
+    Foo(x, *y)
+}
+
+// CHECK: attributes #0 = {{{.*}} noreturn {{.*}}}
+
+// CHECK: DISubprogram(name: "empty", {{.*}} DIFlagNoReturn
+// CHECK: DISubprogram(name: "foo", {{.*}} DIFlagNoReturn