[Sema] Look through OpenExistentialExpr in checkIgnoreExpr()

[theblixguy]

This PR fixes an issue where the DiscardableResultAttr would be ignored if the sub-expression of an OptionalEvaluationExpr was an OpenExistentialExpr.

Example:

protocol FooProtocol {}

extension FooProtocol {
  @discardableResult
  static func returnSomething() -> Bool? {
    return true
  }
}

class Foo {
  var myOptionalFooProtocol: FooProtocol.Type?

  func doSomething() {
    myOptionalFooProtocol?.returnSomething() // Expression of type 'Bool?' is unused
  }
}

Resolves SR-9646.

[theblixguy]

cc @jrose-apple @slavapestov @xedin

[xedin]

LGTM!

I noticed all of indentation commits across your PRs. Here is the workflow I suggest - there is git-clang-format in llvm project, if you compiling your project with -r add following to your PATH - <path-to-swift-directory>/swift/build/Ninja-RelWithDebInfoAssert/llvm-macosx-x86_64/bin so you can do git clang-format <filename>? (for debug (-d) default build directory name is - Ninja-DebugAssert).

When I'm working on something and about to commit I git add all of the modified files and do git clang-format, add re-formatted files and commit.

If you already made a commit but to format code you do git commit --amend and (force) push changes to your branch. You can also use git squash HEAD~2 if you have formatting commit directly after changes.

[xedin]

@theblixguy Want to try squashing re-ident + name update into first commit before we merge?

[theblixguy]

@xedin done :)

[xedin]

@swift-ci please smoke test

[theblixguy]

All tests have passed ✅ @xedin

[jrose-apple]

I wonder if this was originally deliberate, to remind people to check whether the action actually occurred, but that's not actually so useful in practice. Thanks, @theblixguy!