redundant_discardable_let should not produce warning when used with async

[mhosna]

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

Describe the bug

let can not be removed from "async let _ = await foo()" because removing it produces invalid swift syntax

$ swiftlint lint

Produces:

Redundant Discardable Let Violation: Prefer `_ = foo()` over `let _ = foo()` when discarding a result from a function. (redundant_discardable_let)

Environment

• SwiftLint version 0.48.0

Example for RedundantDiscardableLetRule test case:

Example("async let _ = await foo()")

Fix:

return file.match(pattern: "(?<!async\\s)let\\s+_\\b", with: [.keyword, .keyword]).filter { range in

[mhosna]

#4143

some tasks ended with error:
ERROR: Failed to query remote execution capabilities: UNAUTHENTICATED: no authentication metadata found
not sure what that mean..

[jpsim]

@mhosna thanks for the report & fix, the unauthenticated error was unrelated to your work, I was making some changes to our CI infrastructure and things were temporarily broken.

[mhosna]

@jpsim thank you for the merge!