Detect use-after-scope bugs with AddressSanitizer

Enable use-after-scope checks by default when using AddressSanitizer.
They allow to detect incorrect use of stack objects after their scope
have already ended. The detection is based on LLVM lifetime intrinsics.

To facilitate the use of this functionality, the lifetime intrinsics are
now emitted regardless of optimization level if enabled sanitizer makes
use of them.

src/test/ui/sanitizer-use-after-scope.rs

@@ -0,0 +1,18 @@
+// needs-sanitizer-support
+// only-x86_64
+//
+// compile-flags: -Zsanitizer=address
+// run-fail
+// error-pattern: ERROR: AddressSanitizer: stack-use-after-scope
+
+static mut P: *mut usize = std::ptr::null_mut();
+
+fn main() {
+    unsafe {
+        {
+            let mut x = 0;
+            P = &mut x;
+        }
+        std::ptr::write_volatile(P, 123);
+    }
+}