Point to the current box syntax tracking issue

The issue was used for both box syntax as well as placement new.
It got closed due to placement new being unapproved.
So a new one got created for box syntax, yet neither
the unstable book nor feature_gate.rs got updated.
We are doing this now.

Author: est31

Diff for: src/doc/unstable-book/src/language-features/box-syntax.md

@@ -1,8 +1,8 @@
 # `box_syntax`
 
-The tracking issue for this feature is: [#27779]
+The tracking issue for this feature is: [#49733]
 
-[#27779]: https://github.com/rust-lang/rust/issues/27779
+[#49733]: https://github.com/rust-lang/rust/issues/49733
 
 See also [`box_patterns`](language-features/box-patterns.html)
 

Diff for: src/libsyntax/feature_gate.rs

@@ -145,7 +145,7 @@ declare_features! (
     // rustc internal
     (active, rustc_diagnostic_macros, "1.0.0", None, None),
     (active, rustc_const_unstable, "1.0.0", None, None),
-    (active, box_syntax, "1.0.0", Some(27779), None),
+    (active, box_syntax, "1.0.0", Some(49733), None),
     (active, unboxed_closures, "1.0.0", Some(29625), None),
 
     (active, fundamental, "1.0.0", Some(29635), None),

Diff for: src/test/ui/feature-gate-box-expr.stderr

@@ -1,4 +1,4 @@
-error[E0658]: box expression syntax is experimental; you can call `Box::new` instead. (see issue #27779)
+error[E0658]: box expression syntax is experimental; you can call `Box::new` instead. (see issue #49733)
   --> $DIR/feature-gate-box-expr.rs:22:13
    |
 LL |     let x = box 'c'; //~ ERROR box expression syntax is experimental

Diff for: src/test/ui/feature-gate-box_syntax.stderr

@@ -1,4 +1,4 @@
-error[E0658]: box expression syntax is experimental; you can call `Box::new` instead. (see issue #27779)
+error[E0658]: box expression syntax is experimental; you can call `Box::new` instead. (see issue #49733)
   --> $DIR/feature-gate-box_syntax.rs:14:13
    |
 LL |     let x = box 3;