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

Old error output:

    3  |         let _: usize = $f;
       |                -----     ^ expected `usize`, found struct `Baz`
       |                |
       |                expected due to this

New error output:

    3  |         let _: usize = $f;
       |                -----   ^^ expected `usize`, found struct `Baz`
       |                |
       |                expected due to this

compiler/rustc_parse/src/parser/expr.rs

@@ -41,7 +41,7 @@ macro_rules! maybe_whole_expr {
                     let path = path.clone();
                     $p.bump();
                     return Ok($p.mk_expr(
-                        $p.token.span,
+                        $p.prev_token.span,
                         ExprKind::Path(None, path),
                         AttrVec::new(),
                     ));

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

@@ -0,0 +1,11 @@
+macro_rules! foo {
+    ( $f:path ) => {{
+        let _: usize = $f; //~ERROR
+    }};
+}
+
+struct Baz;
+
+fn main() {
+    foo!(Baz);
+}

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

@@ -0,0 +1,16 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-87812-path.rs:3:24
+   |
+LL |         let _: usize = $f;
+   |                -----   ^^ expected `usize`, found struct `Baz`
+   |                |
+   |                expected due to this
+...
+LL |     foo!(Baz);
+   |     ---------- in this macro invocation
+   |
+   = note: this error originates in the macro `foo` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.