Extend dead code lint to detect more unused enum variants, rebased

[sanxiyn]

Rebase of #21468.

Fix #25960.

[rust-highfive]

r? @pcwalton

(rust_highfive has picked a reviewer for you, use r? to override)

[huonw]

Could you add a test like:

enum Foo { A, B }

fn bar() -> Foo {
    match Foo::A {
        Foo::A | Foo::B => Foo::B
    }
}

That is, ensuring that alternation doesn't get cause variants to count as unused.

[alexcrichton]

r? @huonw

[sanxiyn]

Added a test.

[sanxiyn]

Ping?

[sanxiyn]

Another ping.

[huonw]

@bors r+

sorry for the delay

[bors]

📌 Commit 09a4d09 has been approved by huonw

[bors]

⌛ Testing commit 09a4d09 with merge 6169aa2...

[bors]

💔 Test failed - auto-mac-64-nopt-t

[sanxiyn]

Failed in pretty. Fixed.

[sanxiyn]

@bors r=huonw

[bors]

📌 Commit ae0716d has been approved by huonw

[bors]

⌛ Testing commit ae0716d with merge e7a5a1c...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt