This attribute is intended to mean there's a replacement declaration that
should be used instead in Swift code. We already filter out decls with this
attribute in code completion, but were still exposing them in generated
interfaces.
Resolves rdar://problem/62464954
The code goes into its own sub-tree under 'tools' but tests go under 'test',
so that running 'check-swift' will also run all the SourceKit tests.
SourceKit is disabled on non-darwin platforms.