Rollup merge of #135731 - frank-king:feature/pin-borrow, r=eholk,traviscross

Implement parsing of pinned borrows

This PR implements part of #130494.

EDIT: It introduces `&pin mut $place` and `&pin const $place` as sugars for `std::pin::pin!($place)` and its shared reference equivalent, except that `$place` will not be moved when borrowing. The borrow check will be in charge of enforcing places cannot be moved or mutably borrowed since being pinned till dropped.

### Implementation steps:
- [x] parse the `&pin mut $place` and `&pin const $place` syntaxes
- [ ] borrowck of `&pin mut|const`
- [ ] support autoref of `&pin mut|const` when needed

compiler/rustc_ast_pretty/src/pprust/state/expr.rs

@@ -357,6 +357,10 @@ impl<'a> State<'a> {
                 self.word_nbsp("raw");
                 self.print_mutability(mutability, true);
             }
+            ast::BorrowKind::Pin => {
+                self.word_nbsp("pin");
+                self.print_mutability(mutability, true);
+            }
         }
         self.print_expr_cond_paren(
             expr,