Implement black_box using intrinsic

The new implementation allows some `memcpy`s to be optimized away,
so the uninit value in ui/sanitize/memory.rs is constructed directly
onto the return place. Therefore the sanitizer now says that the
value is allocated by `main` rather than `random`.

compiler/rustc_span/src/symbol.rs

@@ -335,6 +335,7 @@ symbols! {
         bitreverse,
         bitxor,
         bitxor_assign,
+        black_box,
         block,
         bool,
         borrowck_graphviz_format,