tools/checklocks: support field comments for guard specifications

[copybara-service]

tools/checklocks: support field comments for guard specifications

This allows specifying guards in the format:

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

This syntax, while more limited, makes writing lock guards less verbose for structures where field documentation isn't attached to each field. For example, being able to write something like the following with field comments reduces the number of lines in the struct by >25% and is, in my opinion, easier to read.

type widget struct {
    mu sync.Mutex // guards following

    // The following counters track [...]

    success  int // +checklocks:mu
    failure  int // +checklocks:mu
    retries  int // +checklocks:mu
    timeouts int // +checklocks:mu
}

FUTURE_COPYBARA_INTEGRATE_REVIEW=#10575 from andrew-d:andrew/checklocks-field-comment e3db6f0