This script improves several things:
* Previous it was incredilby easy to accidently mis-use the script. For
example running `build-toolchain -n` would set `BUNDLE_PREFIX` to `-n`.
The new implementation of command line argument parsing is less fragile
and easier to extend.
* Provides `--help` (or `-h`) output.
* Prints the value of several environment variables before running
`build-script`.
* Shows the invocation of `build-script`.
The accepted order of command line arguments has not changed to avoid
breaking existing users of the script.
If -n or --dry-run is specified in command line arguments, print the commands
that would be executed to stdout, but do not execute them.
Supported in build-script and build-toolchain.
utils/build-script -n -Rt
utils/build-script -n --preset=buildbot_incremental,tools=RA,stdlib=RA
utils/build-toolchain -n local.swift