Enable module builds of llvm/clang by default

[vedantk]

What's in this pull request?

Add an option to build-script-impl to enable building llvm/clang with modules. This supersedes PR 3096.

Resolved bug number: (SR-)

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Before merging this pull request to apple/swift repository:

• Test pull request on Swift continuous integration.

Triggering Swift CI

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Smoke Testing

Platform	Comment
All supported platforms	@swift-ci Please smoke test
All supported platforms	@swift-ci Please smoke test and merge
OS X platform	@swift-ci Please smoke test OS X platform
Linux platform	@swift-ci Please smoke test Linux platform

A smoke test on macOS does the following:

1. Builds the compiler incrementally.
2. Builds the standard library only for macOS. Simulator standard libraries and
   device standard libraries are not built.
3. lldb is not built.
4. The test and validation-test targets are run only for macOS. The optimized
   version of these tests are not run.

A smoke test on Linux does the following:

1. Builds the compiler incrementally.
2. Builds the standard library incrementally.
3. lldb is built incrementally.
4. The swift test and validation-test targets are run. The optimized version of these
   tests are not run.
5. lldb is tested.

Validation Testing

Platform	Comment
All supported platforms	@swift-ci Please test
All supported platforms	@swift-ci Please test and merge
OS X platform	@swift-ci Please test OS X platform
OS X platform	@swift-ci Please benchmark
Linux platform	@swift-ci Please test Linux platform

Lint Testing

Language	Comment
Python	@swift-ci Please Python lint

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[vedantk]

@swift-ci Please smoke test

[vedantk]

This failed because utils/update-checkout on master-next was not new enough. I cherry-picked 0fa1226 and 68661f9. @swift-ci Please smoke test

[vedantk]

Cmake goop unrelated to this PR is blowing up, but only on Linux -- macOS passes all tests :|. Guess I'll try it again with a different bot config: @swift-ci Please test Linux platform

[vedantk]

OK. The full linux test suite passed too. It's still a problem that the smoke tests didn't pass. I'll try it again: @swift-ci Please smoke test Linux platform

[gottesmm]

@swift-ci Please smoke test linux platform

[adrian-prantl]

@shahmishal Do the smoke tests clear the build directory or are they incremental?
@vedantk If they are incremental, I would not be surprised if this fails with random errors. Changing the CMake configuration without erasing the CMake cache is not supported.

[gottesmm]

@adrian-prantl Please read the documents in docs/ContinuousIntegration.md

[vedantk]

It looks like the smoke tests are in fact incremental. Where does that leave us? Since the validation tests passed, is it OK to merge this in?

[gottesmm]

Master branch is locked. So we wait until Mishal gives the go ahead.

[adrian-prantl]

@gottesmm: Thanks for pointing me to the right document. According to the document test and smoke test are incremental, so we were getting lucky.
This pull request is for master-next, not for master.

[shahmishal]

@adrian-prantl llvm and clang are incremental, however swift is clean build.

Also, CI is ready to accept new PR testing and repository is unlocked.