Suggest unwrapping when private method name is available in inner type

Given

```rust
fn main() {
    let maybe_vec = Some(vec![1,2,3]);
    assert_eq!(maybe_vec.len(), 3);
}
```

suggest unwraping `maybe_vec` to call `.len()` on the `Vec<_>`.

```
error[E0624]: method `len` is private
  --> $DIR/enum-method-probe.rs:61:9
   |
LL |     res.len();
   |         ^^^ private method
  --> $SRC_DIR/core/src/option.rs:LL:COL
   |
   = note: private method defined here
   |
note: the method `len` exists on the type `Vec<{integer}>`
  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
help: consider using `Option::expect` to unwrap the `Vec<{integer}>` value, panicking if the value is an `Option::None`
   |
LL |     res.expect("REASON").len();
   |        +++++++++++++++++
```

Author: estebank

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -264,6 +264,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     err.span_label(within_macro_span, "due to this macro variable");
                 }
                 self.suggest_valid_traits(&mut err, item_name, out_of_scope_traits, true);
+                self.suggest_unwrapping_inner_self(&mut err, source, rcvr_ty, item_name);
                 err.emit()
             }
 

tests/ui/suggestions/enum-method-probe.fixed

@@ -56,4 +56,11 @@ fn test_option_in_unit_return() {
     //~| HELP consider using `Option::expect` to unwrap the `Foo` value, panicking if the value is an `Option::None`
 }
 
+fn test_option_private_method() {
+    let res: Option<_> = Some(vec![1, 2, 3]);
+    res.expect("REASON").len();
+    //~^ ERROR method `len` is private
+    //~| HELP consider using `Option::expect` to unwrap the `Vec<{integer}>` value, panicking if the value is an `Option::None`
+}
+
 fn main() {}

tests/ui/suggestions/enum-method-probe.rs

@@ -56,4 +56,11 @@ fn test_option_in_unit_return() {
     //~| HELP consider using `Option::expect` to unwrap the `Foo` value, panicking if the value is an `Option::None`
 }
 
+fn test_option_private_method() {
+    let res: Option<_> = Some(vec![1, 2, 3]);
+    res.len();
+    //~^ ERROR method `len` is private
+    //~| HELP consider using `Option::expect` to unwrap the `Vec<{integer}>` value, panicking if the value is an `Option::None`
+}
+
 fn main() {}

tests/ui/suggestions/enum-method-probe.stderr

@@ -94,6 +94,23 @@ help: consider using `Option::expect` to unwrap the `Foo` value, panicking if th
 LL |     res.expect("REASON").get();
    |        +++++++++++++++++
 
-error: aborting due to 6 previous errors
+error[E0624]: method `len` is private
+  --> $DIR/enum-method-probe.rs:61:9
+   |
+LL |     res.len();
+   |         ^^^ private method
+  --> $SRC_DIR/core/src/option.rs:LL:COL
+   |
+   = note: private method defined here
+   |
+note: the method `len` exists on the type `Vec<{integer}>`
+  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
+help: consider using `Option::expect` to unwrap the `Vec<{integer}>` value, panicking if the value is an `Option::None`
+   |
+LL |     res.expect("REASON").len();
+   |        +++++++++++++++++
+
+error: aborting due to 7 previous errors
 
-For more information about this error, try `rustc --explain E0599`.
+Some errors have detailed explanations: E0599, E0624.
+For more information about an error, try `rustc --explain E0599`.