[5.8][build] Make sure SPM is built with the forked clang, not the host clang

[finagolfin]

Cherrypick of #64629

Explanation: SPM was built with the freshly-built Swift toolchain, including the Swift-forked clang, for years, but a change in swiftlang/swift-package-manager#5894 late last year inadvertently switched it to be built with the system clang used to natively build the Swift compiler, ie self.toolchain.cc in build-script. That passed the CI but caused problems when cross-compiling SPM on my daily Android CI, as the system clang is only meant for native compilation- for example, we use the freshly-built Swift-forked clang for the native host by default when cross-compiling the Swift compiler itself- so I've been using this patch for the last seven months on my Android CI.

Scope: Only affects the C/C++/asm files built as part of SwiftPM, particularly when cross-compiled

Issue: None

Risk: low, as this only affects compilation of non-Swift portions of SPM itself

Testing: Passed all CI on trunk and I've been using this on my Android CI since last year

Reviewer: @etcwilde

[finagolfin]

@kateinoigakukun, would you run the CI on this?

[kateinoigakukun]

@swift-ci Please test

[al45tair]

We’re going to revert the original as it’s broken the SwiftPM option on build-script.

[etcwilde]

@swift-ci please test macOS

[finagolfin]

@al45tair, what about this one? I can submit your SPM change for 5.8 too, then this should be fine.

[al45tair]

Yeah, the SwiftPM change needs cherry picking first. I was going to do that later on today (I'm build wrangler at the moment so I'm busy doing that right now).

[finagolfin]

@al45tair, I can submit that 5.8 SPM pull with your commit for you if you're busy, just let me know.

[al45tair]

The cherry pick of that change is here: swiftlang/swift-package-manager#6754

[finagolfin]

@egorzhdan, one last CI run and we can get this in.

[al45tair]

I don't think we need a CI run so much as a branch manager to approve it. @tomerd you can do that I believe.

[finagolfin]

@al45tair, this pull has not been run through the CI since your SPM pull was merged into 5.8 before the weekend, which is why I suggested it.

[al45tair]

@swift-ci Please test

[finagolfin]

Linux CI failed when checking out the source, it seems super flaky this week.

[al45tair]

@swift-ci Please test Linux platform

[finagolfin]

@bnbarham, one last linux CI run?

[bnbarham]

@swift-ci Please test Linux platform

[finagolfin]

@egorzhdan, one more linux CI run, please.

[egorzhdan]

@swift-ci please test Linux

[finagolfin]

Whoo, it didn't crash at source checkout this time. 😄

[finagolfin]

@tomerd, ready for final approval and merge.