Emit helpful `note` messages only once

[bluss]

Example: Adding #![deny(lint_name)] to a crate will point out the location of this line for every time the lint fires (note: lint level defined here). This should be listed only once. It should be reasonably easy to deduplicate these based upon equal span and message.

src/adaptors.rs:84:5: 87:6 warning: missing documentation for a method
src/adaptors.rs:84     pub fn new(iter: I, map: fn(I::Item) -> B) -> Self
src/adaptors.rs:85     {
src/adaptors.rs:86         FnMap{iter: iter, map: map}
src/adaptors.rs:87     }
src/lib.rs:1:9: 1:21 note: lint level defined here
src/lib.rs:1 #![warn(missing_docs)]
                     ^~~~~~~~~~~~
src/islice.rs:30:5: 37:6 warning: missing documentation for a method
src/islice.rs:30     pub fn new<R: GenericRange>(iter: I, range: R) -> Self
src/islice.rs:31     {
src/islice.rs:32         ISlice {
src/islice.rs:33             start: range.start().unwrap_or(0),
src/islice.rs:34             end: range.end().unwrap_or(::std::usize::MAX),
src/islice.rs:35             iter: iter,
                 ...
src/lib.rs:1:9: 1:21 note: lint level defined here
src/lib.rs:1 #![warn(missing_docs)]
                     ^~~~~~~~~~~~
src/misc.rs:15:5: 15:17 warning: missing documentation for an associated type
src/misc.rs:15     type Result;
                   ^~~~~~~~~~~~
src/lib.rs:1:9: 1:21 note: lint level defined here
src/lib.rs:1 #![warn(missing_docs)]

[mitaa]

I think this is a good "E-easy" candidate.

Relevant code is in src/librustc/lint/context.rs.
Using a HashSet<(Span, &str)> as a field on LintStore should work, so that we only emit a note the first time we encounter it.

[nikomatsakis]

Question -- in #37191, we get only "lint level defined here" per #[allow], even if it applies to multiple lints (e.g., #[allow(warnings)]). Do people think it'd be better to have it like this, or would you rather see different messages for (say) dead_code and unused_variable, even if they both apply to the same #[allow]?