Add support for catching dead recursive code with unused_declaration

Currently if you have code like this:

/tmp/foo.swift:

struct Foo {
    var baz = true

    func bar() {
        if self.baz {
            self.bar()
        }
    }
}

print(Foo())

And you run the unused_declaration rule on it with this config:

.swiftlint.yml:

included:
  - /tmp/foo.swift
analyzer_rules:
  - unused_declaration

commands.json:

[
    {
        "arguments": [
            "swiftc",
            "/tmp/Foo.swift",
            "-sdk",
            "/Applications/Xcode-11.3.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk"
        ],
        "file": "/tmp/Foo.swift"
    }
]

And you run swiftlint:

% swiftlint analyze --compile-commands $PWD/commands.json
Loading configuration from '.swiftlint.yml'
Analyzing Swift files at paths
Collecting 'Foo.swift' (1/1)
Analyzing 'Foo.swift' (1/1)
Done analyzing! Found 0 violations, 0 serious in 1 file.

bar is not diagnosed as being unused because it calls itself. It would be awesome if references included in the same definition they're referenced by were not included when determining if something is unused.

Start with the unused_declaration analyzer and reproduce the example using swiftlint analyze --compile-commands commands.json with the shown .swiftlint.yml and Swift source. Done means the self-recursive bar method is diagnosed as unused without counting its reference from within its own definition.