Add Single Module Build Mode to CMake for Foundation

[jmschonfeld]

This adds two new options to the CMake build: COLLECTIONS_SINGLE_MODULE and COLLECTIONS_FOUNDATION_TOOLCHAIN_MODULE. The former builds the project into one single module and mimics the setup for this mode that the package manifest provides. The later implies a single module and additionally renames the module to _FoundationCollections, adds the rope module's sources to the combined module, and limits the allowed client of the module to FoundationEssentials. This work is in order to support building swift-foundation (which depends on swift-collections) as part of the toolchain.

The cmake files are only used by the toolchain build, but I've validated locally that building a linux toolchain with swift-foundation (which will invoke this cmake build both with and without COLLECTIONS_FOUNDATION_TOOLCHAIN_MODULE) successfully creates a toolchain and that the version with COLLECTIONS_FOUNDATION_TOOLCHAIN_MODULE creates a single library that is statically linked into FoundationEssentials and installs just the swift module in the appropriate location.

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.

[lorentey]

Looks good!