This flag is a GNU extension, and would cause misleading test failures
on other platforms where this extension is not available. However, the
necessity to switch line endings is only required on Windows when
testing. We could use sed to canonicalize line endings before comparing,
but that may cause higher amounts of filesystem traffic on Windows which
would slow down testing.
Instead, move the substitution for diff in SourceKit's lit.local.cfg up
to the top level, and conditionalize the substitution which has the flag
on Windows, but not on other platforms (where it should not be required).
This adjusts the test diffing to ignore the whitespace changes across
different platforms. `raw_fd_ostream` will write out `OF_Text` files
with the platform's line endings, which may differ from the source
file's.
We were trying to get fancy rewriting cases where multiple 'if let' clauses were
used to unwrap a double optional, but in testing that turned out to be too
unreliable. Now we simply detect any 'try?' expressions whose behavior will change,
and add an explicit 'as OldType' to the expression, in order to preserve the original
behavior.
