Account for bare tuples in field searching logic

When looking for the field names and types of a given type, account for tuples. This allows suggestions for incorrectly nested field accesses and field name typos to trigger as intended. Previously these suggestions only worked on `ty::Adt`, including tuple structs which are no different to tuples, so they should behave the same in suggestions.

```
error[E0599]: no method named `get_ref` found for tuple `(BufReader<File>,)` in the current scope
  --> $DIR/missing-field-access.rs:11:15
   |
LL |     let x = f.get_ref();
   |               ^^^^^^^ method not found in `(BufReader<File>,)`
   |
help: one of the expressions' fields has a method of the same name
   |
LL |     let x = f.0.get_ref();
   |               ++
```

1 files changed, 4 insertions, 0 deletions

diff --git a/tests/ui/traits/well-formed-recursion-limit.stderr b/tests/ui/traits/well-formed-recursion-limit.stderr
index e0270ecabbd..a4c85c4fcbd 100644
--- a/tests/ui/traits/well-formed-recursion-limit.stderr
+++ b/tests/ui/traits/well-formed-recursion-limit.stderr
@@ -3,12 +3,16 @@ error[E0609]: no field `ab` on type `(Box<(dyn Fn(Option<A>) -> Option<B> + 'sta
    |
 LL |     let (ab, ba) = (i.ab, i.ba);
    |                       ^^ unknown field
+   |
+   = note: available fields are: `0`, `1`
 
 error[E0609]: no field `ba` on type `(Box<(dyn Fn(Option<A>) -> Option<B> + 'static)>, Box<(dyn Fn(Option<B>) -> Option<A> + 'static)>)`
   --> $DIR/well-formed-recursion-limit.rs:12:29
    |
 LL |     let (ab, ba) = (i.ab, i.ba);
    |                             ^^ unknown field
+   |
+   = note: available fields are: `0`, `1`
 
 error[E0275]: overflow assigning `_` to `Option<_>`
   --> $DIR/well-formed-recursion-limit.rs:15:33