The warnings that ClangImporter emits about issues it encounters while
importing declarations from Clang modules should all belong to a diagnostic
group so that users of `-warnings-as-errors` can control their behavior using
the compiler flags introduce with SE-0443. It's especially important that these
diagnostics be controllable since they are often caused by external
dependencies and therefore the developer may not have any control over whether
they are emitted.
The `#ClangDeclarationImport` diagnostic group is intentionally broad so that
developers have a way to control all of these diagnostics with a single
`-Wwarning` flag. I fully expect that we'll introduce finer-grained diagnostic
groups for some of these diagnostics in the future, but those groups should be
hierarchically nested under `#ClangDeclarationImport`, which is supported by
SE-0443.
Resolves rdar://150524204.
This can lead to latent type errors for API users,
because a swiftmodule would otherwise be emitted,
without any diagnostics, containing imported decl
with two global actor annotations on it. Such
decls will always be an error to the typechecker
when its eventually encountered.
This patch drops all `@MainActor` annotations after
the first one in the ClangImporter, regardless of
whether its the safe or unsafe version, and emits
a warning when doing so.
