build-script: fix --skip-build-llvm not having full effect

[MaxDesiatov]

While LLVM cannot be completely disabled when Swift is built, we should still allow skipping LLVM configuration and build steps when Swift is not built. This can happen, for example, when build-script user wants an ad-hoc incremental build with --skip-build-swift for products that don't depend on LLVM and Swift, or if LLVM and Swift were already configured and built in previous build-script invocations.

[MaxDesiatov]

@swift-ci smoke test

[bnbarham]

Seems reasonable to me. Do we normally use the just-built clang or host clang when building everything (that is C/C++) else?

[MaxDesiatov]

Seems reasonable to me. Do we normally use the just-built clang or host clang when building everything (that is C/C++) else?

Just built, same as Swift. That's why making it conditional on --skip-build-swift makes sense IMO.

[MaxDesiatov]

@swift-ci smoke test

[etcwilde]

I think this is okay. (Can you check that the lldb product builds correctly standalone/doesn't depend on the required pieces of the LLVM product)
@drexin wrote the build-script product for LLVM though, so he might have better recollection about why it was done the way that it was.

[drexin]

I think this is okay. (Can you check that the lldb product builds correctly standalone/doesn't depend on the required pieces of the LLVM product) @drexin wrote the build-script product for LLVM though, so he might have better recollection about why it was done the way that it was.

The comment above basically explains it, but I also only reproduced what build-script-impl did.