Fix borrow check warning
Summary:
Previously we had the following warning:
```
warning: cannot borrow `*self` as mutable because it is also borrowed as immutable
--> hphp/hack/src/parser/lexer.rs:1956:25
|
1946 | let original_text = self.current_text_as_str(); //
| ---- immutable borrow occurs here
...
1956 | self.with_error(Errors::uppercase_kw(original_text));
| ^^^^ mutable borrow occurs here ------------- immutable borrow later used here
|
= note: #[warn(mutable_borrow_reservation_conflict)] on by default
= warning: this borrowing pattern was not meant to be accepted, and may become a hard error in the future
= note: for more information, see issue #59159 <https://github.com/rust-lang/rust/issues/59159>
```
Refactor the code so that the scope of `original_text` doesn't extend to `self.with_error`. This is consistent with the vector example given in [Rust issue #59159](https://github.com/rust-lang/rust/issues/59159).
Reviewed By: losvald
Differential Revision:
D15759079
fbshipit-source-id:
f5d067f32243abc75c7d2ac35709628ea34a48e9