Fixes to manual_let_else's divergence check

[Jarcho]

A few changes to the divergence check in manual_let_else and moves it the implementation to clippy_utils since it's generally useful:

• Handle internal break and continue expressions.
  e.g. The first loop is divergent, but the second is not.

  {
      loop {
          break 'outer;
      };
  }
  {
      loop {
          break;
      };
  }

• Match rust's definition of divergence which is defined via the type system.
  e.g. The following is not considered divergent by rustc as the inner block has a result type of ():

  {
      'a: {
          panic!();
          break 'a;
      };
  }

• Handle when adding a single semicolon would make the expression divergent.
  e.g. The following would be a divergent if a semicolon were added after the if expression:

  { if panic!() { 0 } else { 1 } }

changelog: None

[rustbot]

r? @dswij

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[dswij]

Looks great, thanks for this!

[dswij]

@bors r+

[bors]

📌 Commit a44bb07 has been approved by dswij

It is now in the queue for this repository.

[bors]

⌛ Testing commit a44bb07 with merge 6a15f3b...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: dswij
Pushing 6a15f3b to master...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: dswij
Pushing 6a15f3b to master...