Remarks are intended to be enabled via eg. `-R...`, where as
`(add|remove)_predates_concurrency_import` is a diagnostic that's always
output without any `-R` flag. Move it to a warning instead.
Resolves rdar://114207080.
This means that:
1. In test cases where minimal is the default (swift 5 without
-warn-concurrency), I added RUN lines for targeted, complete, and complete +
sns.
2. In test cases where complete is the default (swift 6, -warn-concurrency,
specified complete with -strict-concurrency), I added a send non-sendable run
line.
In each of these cases, I added additional expected-* lines as appropriate so
the tests can compile in each mode successfully.
When compiling with Swift 6 mode, but using a serialized module
that is using Swift 5 that contains a struct with global-actor
isolation on its stored property, we should not emit an error
for that stored property. Only a warning was emitted when compiling
that module, but the checks here were treating that module as being
type-checked again under a Swift 6 world.
