Never inline naked functions

The `#[naked]` attribute disabled prologue / epilogue emission for the
function and it is responsibility of a developer to provide them. The
compiler is no position to inline such functions correctly.

Disable inlining of naked functions at LLVM and MIR level.

src/test/codegen/naked-noinline.rs

@@ -0,0 +1,30 @@
+// Checks that naked functions are never inlined.
+// compile-flags: -O -Zmir-opt-level=2
+// ignore-wasm32
+#![crate_type = "lib"]
+#![feature(asm)]
+#![feature(naked_functions)]
+
+#[inline(always)]
+#[naked]
+#[no_mangle]
+pub unsafe extern "C" fn f() {
+// Check that f has naked and noinline attributes.
+//
+// CHECK:       define void @f() unnamed_addr [[ATTR:#[0-9]+]]
+// CHECK-NEXT:  start:
+// CHECK-NEXT:    call void asm
+    asm!("", options(noreturn));
+}
+
+#[no_mangle]
+pub unsafe fn g() {
+// Check that call to f is not inlined.
+//
+// CHECK-LABEL: define void @g()
+// CHECK-NEXT:  start:
+// CHECK-NEXT:    call void @f()
+    f();
+}
+
+// CHECK: attributes [[ATTR]] = { naked noinline{{.*}} }