adjust error messages

Author: RalfJung

tests/compile-fail/validity/invalid_enum_tag.rs

@@ -4,5 +4,5 @@ pub enum Foo {
 }
 
 fn main() {
-    let _f = unsafe { std::mem::transmute::<i32, Foo>(42) }; //~ ERROR encountered 0x0000002a, but expected a valid enum tag
+    let _f = unsafe { std::mem::transmute::<i32, Foo>(42) }; //~ ERROR encountered 0x0000002a at .<enum-tag>, but expected a valid enum tag
 }

tests/compile-fail/validity/invalid_enum_tag_256variants_uninit.rs

@@ -266,5 +266,5 @@ union MyUninit {
 }
 
 fn main() {
-    let _a = unsafe { MyUninit { init: () }.uninit }; //~ ERROR encountered uninitialized bytes, but expected a valid enum tag
+    let _a = unsafe { MyUninit { init: () }.uninit }; //~ ERROR encountered uninitialized bytes at .<enum-tag>, but expected a valid enum tag
 }

tests/compile-fail/validity/transmute_through_ptr.rs

@@ -10,5 +10,5 @@ fn main() {
     let mut x = Bool::True;
     evil(&mut x);
     let _y = x; // reading this ought to be enough to trigger validation
-    //~^ ERROR encountered 0x0000002c, but expected a valid enum tag
+    //~^ ERROR encountered 0x0000002c at .<enum-tag>, but expected a valid enum tag
 }