When the CustomAvailability experimental feature is enabled, make it an error
to specify an unrecognized availability domain name. Also, add these
diagnostics to a diagnostic group so that developers can control their behavior
when they are warnings.
Resolves rdar://152741624.
When serializing `@available` attributes, if the attribute applies to a custom
domain include enough information to deserialize the reference to that domain.
Resolves rdar://138441265.
Partially revert https://github.com/swiftlang/swift/pull/80035 now that Clang
has its own APIs for querying serialized modules for the decl representing the
availability domain with a given name.
The `#import` directive is not supported by Clang in MSVC compatibility mode,
so use `#include` instead for compatibility in the ClangImporter tests for
custom availability domains.
Resolves rdar://147122406.
In addition to tracking availability domains in SwiftLookupTable, also
serialize and deserialize the mapping from domain name to `clang::VarDecl`.
Ideally this serialization and lookup infrastructure would be entirely handled
by Clang, since it also needs to look up availability domains in serialized
modules, but the implementation for that is not ready yet.
Part of rdar://138441266.
