Add #[rustc_clean(loaded_from_disk)] to assert loading of query result

Currently, you can use `#[rustc_clean]` to assert to that a particular
query (technically, a `DepNode`) is green or red. However, a green
`DepNode` does not mean that the query result was actually deserialized
from disk - we might have never re-run a query that needed the result.

Some incremental tests are written as regression tests for ICEs that
occured during query result decoding. Using
`#[rustc_clean(loaded_from_disk="typeck")]`, you can now assert
that the result of a particular query (e.g. `typeck`) was actually
loaded from disk, in addition to being green.

compiler/rustc_span/src/symbol.rs

@@ -788,6 +788,7 @@ symbols! {
         literal,
         llvm_asm,
         load,
+        loaded_from_disk,
         local,
         local_inner_macros,
         log10f32,