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.
Some test now fail, so add an explicit -swift-version 3.
Apparently, ReplaceText() on the same location replaces the replaced text. Using InsertText() fixes the problem. rdar://problem/39518867
