Prepare for introducing swift-foundation into the toolchain

[jmschonfeld]

Per @parkera's update here, we will be adding swift-foundation to the toolchain and updating the swift-corelibs-foundation in the toolchain to be "re-cored" on this new implementation. This adds the initial steps to landing these changes:

1. Update the list of checkouts to include the swift-foundation and swift-foundation icu repos
2. Passing extra variables down to the cmake invocation of swift-corelibs-foundation so that it can appropriately find the new dependencies it will have (swift-collections, swift-syntax, swift-foundation-icu, and swift-foundation)

This change shouldn't affect the build itself today, but will allow us to introduce changes into swift-corelibs-foundation that utilize these new libraries.

[jmschonfeld]

@swift-ci please smoke test

[kateinoigakukun]

Seems like we need to update CI system configurations to check out those new repositories. Some jobs on the main branch are currently failing due to missing checked outs repos. CC: @shahmishal
https://ci.swift.org/job/oss-swift-RA-linux-ubuntu-22_04/7404/console

[finagolfin]

@shahmishal and @parkera, now that swift-foundation was pushed into swift-corelibs-foundation, swiftlang/swift-corelibs-foundation#5008, these two repos will need snapshot tags set up. Otherwise, a tag checkout on the 6.0/main branches will check out an arbitrary commit of swift-foundation and swift-foundation-icu.