https://bugs.swift.org/browse/SR-237 calls for `build-script` and
`build-script-impl` to be merged. This commit takes another step towards
that goal by moving the logic that finds the path to the `clang` and
`clang++` executables up into Python-land.
Rather than simply moving all of the logic into `utils/build-script`,
this commit moves relevant functions into a new Python module, named
`swift_build_support`. This has several benefits:
- The logic can be tested. Whereas `build-script-impl` needed to be run
in order to verify its behavior, the logic extracted out of it into
`swift_build_support` can be tested in isolation.
- The logic can be split up into several files without polluting the
`utils` directory, which now contains many different files that are
unrelated to `build-script`.
