Display \t in diagnostics code as four spaces

src/test/ui/codemap_tests/tab.stderr

@@ -1,16 +1,16 @@
 error[E0425]: cannot find value `bar` in this scope
   --> $DIR/tab.rs:14:2
    |
-14 |  bar; //~ ERROR cannot find value `bar`
-   |  ^^^ not found in this scope
+14 |     bar; //~ ERROR cannot find value `bar`
+   |     ^^^ not found in this scope
 
 error[E0308]: mismatched types
   --> $DIR/tab.rs:18:2
    |
 17 | fn foo() {
    |          - help: try adding a return type: `-> &'static str `
-18 |  "bar   boo" //~ ERROR mismatched types
-   |  ^^^^^^^^^^^ expected (), found reference
+18 |     "bar            boo" //~ ERROR mismatched types
+   |     ^^^^^^^^^^^^^^^^^^^^ expected (), found reference
    |
    = note: expected type `()`
               found type `&'static str`

src/test/ui/codemap_tests/tab_2.stderr

@@ -1,8 +1,8 @@
 error: unterminated double quote string
   --> $DIR/tab_2.rs:14:7
    |
-14 |       """; //~ ERROR unterminated double quote
-   |  _______^
+14 |                   """; //~ ERROR unterminated double quote
+   |  ___________________^
 15 | | }
    | |__^
 

src/test/ui/codemap_tests/tab_3.stderr

@@ -1,11 +1,11 @@
 error[E0382]: use of moved value: `some_vec`
   --> $DIR/tab_3.rs:17:20
    |
-15 |  some_vec.into_iter();
-   |  -------- value moved here
-16 |  {
-17 |   println!("{:?}", some_vec); //~ ERROR use of moved
-   |                    ^^^^^^^^ value used here after move
+15 |     some_vec.into_iter();
+   |     -------- value moved here
+16 |     {
+17 |         println!("{:?}", some_vec); //~ ERROR use of moved
+   |                          ^^^^^^^^ value used here after move
    |
    = note: move occurs because `some_vec` has type `std::vec::Vec<&str>`, which does not implement the `Copy` trait