Re-apply PR #1340

[glessard]

Re-apply #1340 and swiftlang/swift-foundation-icu#63 when it is possible to do so.

They depend on swiftlang/swift@c08372a.

We should re-apply them once the compiler used by our Windows CI includes the commit above.

[glessard]

This will undo #1414

[glessard]

cc @compnerd

[compnerd]

@iCharlesHu @parkera - I think we should pause all ICU commits and Localization changes until this is restored so that we don't regress the change (or would Apple be taking up the work to enable the early swift-driver on Windows at this point?).

CC: @egorzhdan @artemcm @etcwilde @jmschonfeld

[parkera]

Can you help me understand what went wrong with the order of the changes here?

[parkera]

It sounds like we need the Windows CI to be new enough - but I'm not sure if the cadence of releases of that have improved.

[compnerd]

I think that we should figure out how to ensure that this type of issue is caught earlier - the changes did go through on the CI. Right now, we just need a new snapshot of the toolchain that has the improved ClangImporter to get the tests to pass. In the mean time, the continued changes to use more ICU APIs tends to regress the build with the macro based aliases (which is what I am concerned about).

The urgency here is that without the symbol renaming for ICU we tend to crash in Foundation due to ABI clashes between the system provided ICU and the modified ICU that Foundation uses (this primarily comes up in the static linked version as in static linking the system version takes precedence).

[parkera]

Totally understand about the desire to not let this bit rot.

In the short term (next few weeks) I'm not expecting us to do much in the way of changes in those areas, because we're focused on wrapping up the 6.2 release.

[compnerd]

Awesome! That will hopefully give us enough time to update the toolchain snapshot that we are using and get this merged back in. At that point, we should be able to hopefully continue the work to enable the early swift driver on Windows.

[compnerd]

In the mean time, the patches that we will need for that are:
swiftlang/swift-foundation-icu#66
#1421