MIR-borrowck: Some minor fixes

[zilbuz]

• Remove parens when printing dereference (fix MIR-borrowck: dereferences are printed wrapped in parens  #45185)
• Change argument type of autoderef to bool
• Change argument type of field_index to Field

[arielb1]

@bors r+

[bors]

📌 Commit 4beca6a has been approved by arielb1

[bors]

⌛ Testing commit 4beca6aad36086532078c205e97dad9d8526272e with merge 98e3158b7619099a2debf23ba36fa43af16e8f86...

[eddyb]

@bors retry (giving priority to #45225)

[bors]

⌛ Testing commit 4beca6aad36086532078c205e97dad9d8526272e with merge 99559815fe2011b10892b9666aa4fbea548ddff3...

[bors]

💔 Test failed - status-travis

[kennytm]

UI test failure in ui/nll/get_default.rs, some left out fixes regarding #45185.

 error[E0502]: cannot borrow `*map` as mutable because it is also borrowed as immutable (Ast)
   --> $DIR/get_default.rs:33:17
    |
 28 |         match map.get() {
    |               --- immutable borrow occurs here
 ...
 33 |                 map.set(String::new()); // Just AST errors here
    |                 ^^^ mutable borrow occurs here
 ...
 37 | }
    | - immutable borrow ends here
 
 error[E0502]: cannot borrow `*map` as mutable because it is also borrowed as immutable (Ast)
   --> $DIR/get_default.rs:43:17
    |
 41 |         match map.get() {
    |               --- immutable borrow occurs here
 42 |             Some(v) => {
 43 |                 map.set(String::new()); // Both AST and MIR error here
    |                 ^^^ mutable borrow occurs here
 ...
 51 | }
    | - immutable borrow ends here
 
 error[E0502]: cannot borrow `*map` as mutable because it is also borrowed as immutable (Ast)
   --> $DIR/get_default.rs:47:17
    |
 41 |         match map.get() {
    |               --- immutable borrow occurs here
 ...
 47 |                 map.set(String::new()); // Just AST errors here
    |                 ^^^ mutable borrow occurs here
 ...
 51 | }
    | - immutable borrow ends here
 
-error[E0502]: cannot borrow `(*map)` as mutable because it is also borrowed as immutable (Mir)
+error[E0502]: cannot borrow `*map` as mutable because it is also borrowed as immutable (Mir)
   --> $DIR/get_default.rs:43:17
    |
 41 |         match map.get() {
    |               --- immutable borrow occurs here
 42 |             Some(v) => {
 43 |                 map.set(String::new()); // Both AST and MIR error here
    |                 ^^^ mutable borrow occurs here
 
 error: aborting due to 4 previous errors

[nikomatsakis]

@bors r+

[bors]

📌 Commit 2285e35 has been approved by nikomatsakis

[bors]

⌛ Testing commit 2285e35 with merge 96e9cee...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing 96e9cee to master...