Handle None-delimited groups when parsing macro_rules! macro

When a `macro_rules!` macro expands to another `macro_rules!` macro, we
may see `None`-delimited groups in odd places when another crate
deserializes the 'inner' macro. This commit 'unwraps' an outer
`None`-delimited group to avoid breaking existing code.

See https://github.com/rust-lang/rust/pull/73569#issuecomment-650860457
for more details.

The proper fix is to handle `None`-delimited groups systematically
throughout the parser, but that will require significant work. In the
meantime, this hack lets us fix important hygiene bugs in macros

src/test/ui/proc-macro/auxiliary/meta-delim.rs

@@ -0,0 +1,12 @@
+macro_rules! produce_it {
+    ($dollar_one:tt $foo:ident $my_name:ident) => {
+        #[macro_export]
+        macro_rules! meta_delim {
+            ($dollar_one ($dollar_one $my_name:ident)*) => {
+                stringify!($dollar_one ($dollar_one $my_name)*)
+            }
+        }
+    }
+}
+
+produce_it!($my_name name);

src/test/ui/proc-macro/meta-delim.rs

@@ -0,0 +1,12 @@
+// aux-build:meta-delim.rs
+// edition:2018
+// run-pass
+
+// Tests that we can properly deserialize a macro with strange delimiters
+// See https://github.com/rust-lang/rust/pull/73569#issuecomment-650860457
+
+extern crate meta_delim;
+
+fn main() {
+    assert_eq!("a bunch of idents", meta_delim::meta_delim!(a bunch of idents));
+}