Rollup merge of #161370 - Manishearth:rustfmt-macro-fix, r=ytmimi

Uplift rustfmt macro formatting fix

This uplifts the fix for rust-lang/rustfmt#6985 (rust-lang/rustfmt#6990), which is a rustfmt ICE breaking multiple crates.

This should be backported to beta.

Author: jhpratt

src/tools/rustfmt/src/macros.rs

@@ -454,7 +454,7 @@ pub(crate) fn rewrite_macro_def(
     };
 
     let mut header = if def.macro_rules {
-        let pos = context.snippet_provider.span_after(span, "macro_rules!");
+        let pos = context.snippet_provider.span_after(span, "!");
         vec![HeaderPart::new("macro_rules!", span.with_hi(pos))]
     } else {
         let macro_lo = context.snippet_provider.span_before(span, "macro");