[Build System: build-script] Adds a new xcrun module to build_swift.

[Rostepher]

This new module replaces the existing module from swift_build_support and is fully tested (including for Python 3).

[Rostepher]

@swift-ci please test

[davezarzycki]

This broke Fedora Linux (perhaps a python 2.7 versus 3 bug?)

FAIL: Swift(linux-x86_64) :: Python/build_swift.swift (510 of 12463)
******************** TEST 'Swift(linux-x86_64) :: Python/build_swift.swift' FAILED ********************
Script:
--
: 'RUN: at line 10';   /usr/bin/python2.7 /home/dave/s/u/swift/utils/build_swift/run_tests.py
--
Exit Code: 1

Command Output (stderr):
--
Traceback (most recent call last):
  File "/home/dave/s/u/swift/utils/build_swift/run_tests.py", line 19, in <module>
    import argparse
  File "/home/dave/s/u/swift/utils/build_swift/argparse/__init__.py", line 18, in <module>
    
ImportError: cannot import name ArgumentDefaultsHelpFormatter

--

[davezarzycki]

Never mind. This seems to be some kind of compiled python cache bug that goes away after git clean -fdx. I wish this cache was kept in the build directory instead of the source directory, but I digress.