Test that stmt_expr_attrs properly gates if-attrs

Author: Aaron1011

src/test/ui/parser/if-attrs/stmt-expr-gated.rs

@@ -0,0 +1,6 @@
+fn main() {
+    let _ = #[deny(warnings)] if true { //~ ERROR attributes on expressions
+    } else if false {
+    } else {
+    };
+}

src/test/ui/parser/if-attrs/stmt-expr-gated.stderr

@@ -0,0 +1,12 @@
+error[E0658]: attributes on expressions are experimental
+  --> $DIR/stmt-expr-gated.rs:2:13
+   |
+LL |     let _ = #[deny(warnings)] if true {
+   |             ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
+   = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.