Checklocks does not support anonymous structs

[Ghanan]

Checklocks does not support anonymous structs

type foo struct {
    bar struct {
        mu sync.RWMutex
        // +checklocks:mu
        x int
    }
}

[Bhasker Hariharan]

Sending this to Adin as he probably has more context on whether this is something we can support in the analyzer.