Properly handle SyntaxContext of dummy spans in incr comp

Fixes #80336

Due to macro expansion, we may end up with spans with an invalid
location and non-root `SyntaxContext`. This commits preserves the
`SyntaxContext` of such spans in the incremental cache, and ensures
that we always hash the `SyntaxContext` when computing the `Fingerprint`
of a `Span`

Previously, we would discard the `SyntaxContext` during serialization to
the incremental cache, causing the span's `Fingerprint` to change across
compilation sessions.

compiler/rustc_span/src/lib.rs

@@ -1899,8 +1899,9 @@ where
             return;
         }
 
-        if *self == DUMMY_SP {
+        if self.is_dummy() {
             Hash::hash(&TAG_INVALID_SPAN, hasher);
+            self.ctxt().hash_stable(ctx, hasher);
             return;
         }