fix(rustc_parse): incorrect span information for macro block expr

Old error output:

   = note: this warning originates in the macro `foo` (in Nightly builds, run with -Z macro-backtrace for more info)
help: wrap this expression in parentheses
   |
4  |             break '_l $f(;)
   |                         ^ ^

New error output:

   = note: this warning originates in the macro `foo` (in Nightly builds, run with -Z macro-backtrace for more info)
help: wrap this expression in parentheses
   |
4  |             break '_l ($f);
   |                       ^  ^

compiler/rustc_parse/src/parser/expr.rs

@@ -50,7 +50,7 @@ macro_rules! maybe_whole_expr {
                     let block = block.clone();
                     $p.bump();
                     return Ok($p.mk_expr(
-                        $p.token.span,
+                        $p.prev_token.span,
                         ExprKind::Block(block, None),
                         AttrVec::new(),
                     ));

src/test/ui/parser/issue-87812.rs

@@ -0,0 +1,13 @@
+#![deny(break_with_label_and_loop)]
+
+macro_rules! foo {
+    ( $f:block ) => {
+        '_l: loop {
+            break '_l $f; //~ERROR
+        }
+    };
+}
+
+fn main() {
+    let x = foo!({ 3 });
+}

src/test/ui/parser/issue-87812.stderr

@@ -0,0 +1,22 @@
+error: this labeled break expression is easy to confuse with an unlabeled break with a labeled value expression
+  --> $DIR/issue-87812.rs:6:13
+   |
+LL |             break '_l $f;
+   |             ^^^^^^^^^^^^
+...
+LL |     let x = foo!({ 3 });
+   |             ----------- in this macro invocation
+   |
+note: the lint level is defined here
+  --> $DIR/issue-87812.rs:1:9
+   |
+LL | #![deny(break_with_label_and_loop)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: this error originates in the macro `foo` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: wrap this expression in parentheses
+   |
+LL |             break '_l ($f);
+   |                       +  +
+
+error: aborting due to previous error
+