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.

src/test/incremental/issue-80336-invalid-span.rs

@@ -0,0 +1,10 @@
+// Regression test for issue #80336
+// Test that we properly handle encoding, decoding, and hashing
+// of spans with an invalid location and non-root `SyntaxContext`
+
+// revisions:rpass1 rpass2
+// only-x86_64
+
+pub fn main() {
+    let _ = is_x86_feature_detected!("avx2");
+}