Fix let_and_return false positive

[phlip9]

The issue:

See this Rust playground link: https://play.rust-lang.org/?edition=2018&gist=12cb5d1e7527f8c37743b87fc4a53748

Run the above with clippy to see the following warning:

warning: returning the result of a `let` binding from a block
  --> src/main.rs:24:5
   |
23 |     let value = Foo::new(&x).value();
   |     --------------------------------- unnecessary `let` binding
24 |     value
   |     ^^^^^
   |
   = note: `#[warn(clippy::let_and_return)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_and_return
help: return the expression directly
   |
23 |
24 |     Foo::new(&x).value()
   |

Implementing the suggested fix, removing the temporary let binding,
yields a compiler error:

error[E0597]: `x` does not live long enough
  --> src/main.rs:23:14
   |
23 |     Foo::new(&x).value()
   |     ---------^^-
   |     |        |
   |     |        borrowed value does not live long enough
   |     a temporary with access to the borrow is created here ...
24 | }
   | -
   | |
   | `x` dropped here while still borrowed
   | ... and the borrow might be used here, when that temporary is dropped and runs the `Drop` code for type `Foo`
   |
   = note: the temporary is part of an expression at the end of a block;
           consider forcing this temporary to be dropped sooner, before the block's local variables are dropped
help: for example, you could save the expression's value in a new local variable `x` and then make `x` be the expression at the end of the block
   |
23 |     let x = Foo::new(&x).value(); x
   |     ^^^^^^^                     ^^^

The fix:

Of course, clippy looks like it should already handle this edge case;
however, it appears utils::fn_def_id is not returning a DefId for
Foo::new. Changing the qpath_res lookup to use the child Path
hir_id instead of the parent Call hir_id fixes the issue.

changelog: none

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @phansch (or someone else) soon.

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[phansch]

@bors r+ thanks!

[bors]

📌 Commit 7f1595e has been approved by phansch

[bors]

⌛ Testing commit 7f1595e with merge f870876...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: phansch
Pushing f870876 to master...