Make let_underscore_drop Deny by default.

This is done so that we can check the noisiness of this lint in a Crater
run. Note that when I built the compiler, I actually encountered lots of
places where this lint will trigger and fail compilation, so I had to
also set `RUSTFLAGS_NOT_BOOSTRAP` to `-A let_underscore_drop` when
compiling to prevent that.

Author: a2aaron

compiler/rustc_lint/src/let_underscore.rs

@@ -46,7 +46,7 @@ declare_lint! {
     /// calling `std::mem::drop` on the expression is clearer and helps convey
     /// intent.
     pub LET_UNDERSCORE_DROP,
-    Allow,
+    Deny,
     "non-binding let on a type that implements `Drop`"
 }