Rollup merge of rust-lang#126101 - lqd:revert-124099, r=wesleywiser

Revert "Disallow ambiguous attributes on expressions" on nightly

As discussed in [today's t-compiler meeting](https://rust-lang.zulipchat.com/#narrow/stream/238009-t-compiler.2Fmeetings/topic/.5Bweekly.5D.202024-06-06/near/443079505), this reverts PR rust-lang#124099 to fix P-critical beta regressions rust-lang#125199.

r? ``@wesleywiser``

Opening as draft so that ``@wesleywiser`` and ``@apiraino,`` you can tell me whether you wanted:
1. a `beta-accepted` revert of rust-lang#124099 on nightly (this PR)? That will need to be backported to beta (even though rust-lang#126093 may be the last of those)
2. a revert of rust-lang#124099 on beta?
3. all of the above?

I also opened rust-lang#126102, another draft PR to revert rust-lang#124099 on beta, should you choose options 2 or 3.

tests/ui/cfg_attr_rustfmt.fixed

@@ -16,7 +16,7 @@ fn foo(
 
 fn skip_on_statements() {
     #[rustfmt::skip]
-    { 5+3; }
+    5+3;
 }
 
 #[rustfmt::skip]
@@ -33,11 +33,11 @@ mod foo {
 #[clippy::msrv = "1.29"]
 fn msrv_1_29() {
     #[cfg_attr(rustfmt, rustfmt::skip)]
-    { 1+29; }
+    1+29;
 }
 
 #[clippy::msrv = "1.30"]
 fn msrv_1_30() {
     #[rustfmt::skip]
-    { 1+30; }
+    1+30;
 }

tests/ui/cfg_attr_rustfmt.rs

@@ -16,7 +16,7 @@ fn foo(
 
 fn skip_on_statements() {
     #[cfg_attr(rustfmt, rustfmt::skip)]
-    { 5+3; }
+    5+3;
 }
 
 #[cfg_attr(rustfmt, rustfmt_skip)]
@@ -33,11 +33,11 @@ mod foo {
 #[clippy::msrv = "1.29"]
 fn msrv_1_29() {
     #[cfg_attr(rustfmt, rustfmt::skip)]
-    { 1+29; }
+    1+29;
 }
 
 #[clippy::msrv = "1.30"]
 fn msrv_1_30() {
     #[cfg_attr(rustfmt, rustfmt::skip)]
-    { 1+30; }
+    1+30;
 }