Use `io.open` to open the files with the correct encoding and to allow
proper newline specification. This allows the script to be python 2 and
python 3 compatible.
Enhances `swift-syntax-test` to output the parser diagnostics so we can verify that
if the incremental parse resulted in parser diagnostics, those diagnostics were also emitted during the full parse.
Use the python library `difflib` which is part of a standard python
installation to generate the diff. This allows us to run the tool on
Windows wihtout needing external tools installed.
