Rework diagnostics for conformance isolation failures

[DougGregor]

A protocol conformance can be ill-formed due to isolation mismatches
between witnesses and requirements, or with associated conformances.
Previously, such failures would be emitted as a number of separate
errors (downgraded to warnings in Swift 5), one for each witness and
potentially an extra for associated conformances. The rest was a
potential flood of diagnostics that was hard to sort through.

Collect all of the isolation-related problems for a given conformance
together and produce a single error (downgraded to a warning when
appropriate) that describes the overall issue. That error will have up
to three notes suggesting specific courses of action:

• Isolating the conformance (when the experimental feature is enabled)
• Marking the witnesses as 'nonisolated' where needed
• Marking the conformance as '@preconcurrency'

The diagnostic also has notes to point out the witnesses/associated
conformances that have isolation problems. There is a new educational
note that also describes these options.

The diagnostic looks like this:

13 | 
14 | @MainActor
15 | final class C: P {
   |             |  |- error: conformance of 'C' to protocol 'P' crosses into main actor-isolated code and can cause data races [#conformance-isolation]
   |             |  |- note: isolate this conformance to the main actor with '@MainActor'
   |             |  `- note: turn data races into runtime errors with '@preconcurrency'
   |             `- note: mark all declarations used in the conformance 'nonisolated'
16 |   var v = 3
   |       `- note: main actor-isolated property 'v' cannot be used to satisfy nonisolated requirement from protocol 'P'
17 |   
18 |   init() {}
19 | 
20 |   func f() { }
   |        `- note: main actor-isolated instance method 'f()' cannot be used to satisfy nonisolated requirement from protocol 'P'
21 |   func g() { }
   |        `- note: main actor-isolated instance method 'g()' cannot be used to satisfy nonisolated requirement from protocol 'P'
22 | }

We give the same treatment to missing 'distributed' on witnesses to a
distributed protocol.

[DougGregor]

@swift-ci please smoke test

[DougGregor]

@swift-ci please smoke test