Fixed: Multiple errors on single typo in match pattern

Here we have fixed the case where we were throwing two diagnostic
messages `E0026` and `E0027` for same case like this

Example
error[E0026]: variant `A::A` does not have a field named `fob`
  --> src/test/ui/issue-52717.rs:20:12
   |
20 |     A::A { fob } => { println!("{}", fob); }
   |            ^^^ variant `A::A` does not have this field

error[E0027]: pattern does not mention field `foo`
  --> src/test/ui/issue-52717.rs:20:5
   |
20 |     A::A { fob } => { println!("{}", fob); }
   |     ^^^^^^^^^^^^ missing field `foo`

error: aborting due to 2 previous errors

Here above we can see that both `E0026` and `E0027` are depicting
same thing.

So, to fix this issue, we are simply checking element of
`inexistent_fields` is there any value lies in
`unmentioned_fields` using Levenshtein algorithm, if does
then for that case we are simply deleting element from
`unmentioned_fields`. More or less now instead of showing
separate message in `E0027` we are giving extra hint on `E0026`

Address: #52717

src/test/ui/issue-52717.rs

@@ -0,0 +1,21 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+enum A {
+  A {
+    foo: usize,
+  }
+}
+
+fn main() {
+  let x = A::A { foo: 3 };
+  match x {
+    A::A { fob } => { println!("{}", fob); }
+  }
+}

src/test/ui/issue-52717.stderr

@@ -0,0 +1,12 @@
+error[E0026]: variant `A::A` does not have a field named `fob`
+  --> $DIR/issue-52717.rs:19:12
+   |
+LL |     A::A { fob } => { println!("{}", fob); }
+   |            ^^^
+   |            |
+   |            variant `A::A` does not have this field
+   |            help: did you mean: `foo`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0026`.

src/test/ui/issues/issue-17800.rs

@@ -17,7 +17,6 @@ fn main() {
     match MyOption::MySome(42) {
         MyOption::MySome { x: 42 } => (),
         //~^ ERROR variant `MyOption::MySome` does not have a field named `x`
-        //~| ERROR pattern does not mention field `0`
         _ => (),
     }
 }

src/test/ui/issues/issue-17800.stderr

@@ -2,17 +2,11 @@ error[E0026]: variant `MyOption::MySome` does not have a field named `x`
   --> $DIR/issue-17800.rs:18:28
    |
 LL |         MyOption::MySome { x: 42 } => (),
-   |                            ^^^^^ variant `MyOption::MySome` does not have this field
+   |                            ^^^^^
+   |                            |
+   |                            variant `MyOption::MySome` does not have this field
+   |                            help: did you mean: `0`
 
-error[E0027]: pattern does not mention field `0`
-  --> $DIR/issue-17800.rs:18:9
-   |
-LL |         MyOption::MySome { x: 42 } => (),
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ missing field `0`
-   |
-   = note: trying to match a tuple variant with a struct variant pattern
+error: aborting due to previous error
 
-error: aborting due to 2 previous errors
-
-Some errors occurred: E0026, E0027.
-For more information about an error, try `rustc --explain E0026`.
+For more information about this error, try `rustc --explain E0026`.