Install the Foundation toolchain module during the static swift build

[jmschonfeld]

This updates the cmake build (specifically the single-module, foundation toolchain build section) to install the _FoundationCollections.a library for the static swift build (but not install it for the non-static build). This ensures the library is present in the static swift SDK in the toolchain (as needed) but not in the standard directory. The usage of this module is still guarded by the -allowable-client flag written into the swift module.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[jmschonfeld]

The toolchain build at https://ci.swift.org/job/swift-PR-toolchain-Linux/873 has confirmed that this no longer poses an issue with the static swift build