Use llvm-ar as default librarian on Unix platforms

[finagolfin]

Resolves #5761

@compnerd, I implemented your TODO comment exactly.

[compnerd]

@swift-ci please smoke test

[compnerd]

Seems like a step in the right direction - @MaxDesiatov thoughts?

[finagolfin]

Sigh, didn't compile this first and had a syntax error. Fixed it now, please rerun the CI.

[MaxDesiatov]

@swift-ci smoke test

[MaxDesiatov]

@swift-ci test windows

[compnerd]

Hmm, this doesn't fallback any longer?

[finagolfin]

this doesn't fallback any longer?

Yes, I removed it based on Boris's suggestion, see the review comments above.

[finagolfin]

@egorzhdan, please run the CI on this.

[finagolfin]

@neonichu, another CI run and we can get this in?

[tomerd]

@swift-ci smoke test

[tomerd]

@MaxDesiatov @compnerd is this good to merge?

[compnerd]

I think that keeping the fallback is better. The librarian is expected to be ar. llvm-ar is a drop-in replacement, but the official archiver is still ar and we should resort to that if necessary.

[MaxDesiatov]

@swift-ci test windows

[MaxDesiatov]

IIRC @al45tair had a relevant change in this area, would be great to get his opinion

[finagolfin]

I don't think the fallback is necessary since the 5.8 toolchains onwards come with llvm-ar, so it just depends how safe you want to play it. @neonichu, wdyt?

The single self-hosted macOS test failure looks like a flake.

[neonichu]

I don't think the fallback is necessary since the 5.8 toolchains onwards come with llvm-ar, so it just depends how safe you want to play it. @neonichu, wdyt?

I agree with you but @compnerd also makes a valid point. It might make new platform bringup easier to keep the fallback in place?

[finagolfin]

OK, I will put the fallback in again.

In the meantime, the macOS smoke test passed, meaning the self-hosted failure was a flake.

[finagolfin]

Rebased and the fallback is back.

[finagolfin]

One last CI run and we can get this in.

[finagolfin]

@egorzhdan, would you run the CI on this?

[egorzhdan]

@swift-ci please test

[finagolfin]

Hmm, only one CI ran?

[neonichu]

@swift-ci please smoke test

[finagolfin]

@compnerd, ready to go in, just needs approval.

[finagolfin]

@neonichu, can we get this in now?

[finagolfin]

@tomerd, can we get this in before the branch tomorrow?

[finagolfin]

@neonichu, would be good to get this merged before the branch today.