Revert "pre-expansion gate try_blocks"

This reverts commit 1935ba658c.

src/libsyntax/feature_gate/check.rs

@@ -518,6 +518,9 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                                        "type ascription is experimental");
                 }
             }
+            ast::ExprKind::TryBlock(_) => {
+                gate_feature_post!(&self, try_blocks, e.span, "`try` expression is experimental");
+            }
             ast::ExprKind::Block(_, opt_label) => {
                 if let Some(label) = opt_label {
                     gate_feature_post!(&self, label_break_value, label.ident.span,

src/test/ui/feature-gates/feature-gate-try_blocks.rs

@@ -1,12 +1,9 @@
 // compile-flags: --edition 2018
 
-#[cfg(FALSE)]
-fn foo() {
-    let try_result: Option<_> = try { //~ ERROR `try` blocks are unstable
+pub fn main() {
+    let try_result: Option<_> = try { //~ ERROR `try` expression is experimental
         let x = 5;
         x
     };
     assert_eq!(try_result, Some(5));
 }
-
-fn main() {}

src/test/ui/feature-gates/feature-gate-try_blocks.stderr

@@ -1,5 +1,5 @@
-error[E0658]: `try` blocks are unstable
-  --> $DIR/feature-gate-try_blocks.rs:5:33
+error[E0658]: `try` expression is experimental
+  --> $DIR/feature-gate-try_blocks.rs:4:33
    |
 LL |       let try_result: Option<_> = try {
    |  _________________________________^