This adjusts the tests for the difference between line endings on
different platforms. Windows uses CRLF while most Unicies use LF. This
was exposed during the update to the new LLVM snapshot.
When sanitizing the documentation comments for synthesized extensions,
we expect some text like "<declaration>extension". This isn't the case
when use-facing attributes are present.
rdar://50913510
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.
