macros: allow Vec fields in diagnostic derive

Diagnostics can have multiple primary spans, or have subdiagnostics
repeated at multiple locations, so support `Vec<..>` fields in the
diagnostic derive which become loops in the generated code.

Signed-off-by: David Wood <david.wood@huawei.com>

src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.rs

@@ -474,3 +474,11 @@ struct Subdiagnostic {
     #[subdiagnostic]
     note: Note,
 }
+
+#[derive(SessionDiagnostic)]
+#[error(code = "E0123", slug = "foo")]
+struct VecField {
+    #[primary_span]
+    #[label]
+    spans: Vec<Span>,
+}