[SR-237][build-script] Migrate Ninja build to Python

[rintaro]

What's in this pull request?

More work on [SR-237] Merge build-script-impl into build-script.

Migrate Ninja build to Python.
Also, some functional changes:

• In OS X, use user configurable toolchain.cxx instead of xcrun --find clang++ to build Ninja.
• When the system installed Ninja is not found, try to build Ninja if --foundation is requested as well as cmake_generator == "Ninja".
• To build Foundation, even without --build-ninja, use build-script detected Ninja path instead of just ninja, because the system installed Ninja can be named ninja-build.
• Added debug trace output for commands used in build-script
  • Clean build directory rm -rf {build_root}
  • Create build directory mkdir -p {build_root}
  • Archive symbols package: tar -c -z -f ...
• Possible regression:
  • Existence check of products source directories has been postponed until after Ninja was built.
  • Report stdlib targets, e.g. Building the standard library for: ..., after Ninja was built.
    (Depends Refactored build-script-impl for cross-compiling support #2497)

Please tell me if these changes are unacceptable.

Migrated impl args:
--build-ninja
--darwin-deployment-version-{osx,ios,tvos,watchos}

Removed impl args:
--build-ninja: Not used in build-script-impl anymore

Added impl args:
--ninja-bin: To propagate just built ninja path to build-script-impl

To make it testable, cherry-picked (and enhanced) dry-runnable shell interface from #2241.

Resolved bug number: (Partially SR-237)

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Before merging this pull request to apple/swift repository:

• Test pull request on Swift continuous integration.

[gribozavr]

The if args.show_sdks: and if args.clean: statements seem to be interrupting the flow here. Is there an ordering dependency? If not, could you reorder this code so that first we show SDKs, then clean, create the build directory, then configure ninja and build it?

[gribozavr]

@swift-ci Please test

[gribozavr]

@rintaro Thank you very much for the change and tests!

[gribozavr]

@swift-ci Please test

[gribozavr]

@rintaro LGTM, just a few small comments.

[rintaro]

Fixed. And added missing xcrun.sdk_path test case.
Omitting stderr=subprocess.PIPE mess up python -m unittest discover output, but it's harmless.

[gribozavr]

@swift-ci Please test