CMake: readd StringProcessing dependency to StdlibUnittest
This reverts commit 4a95275bdedcaaff54875eca71286147758078d2.
I remove the dependency looking at the modules that StdlibUnittest
directly imports -- but it turns out we import StringProcessing
indirectly as a result of importing Foundation from e.g. the underlying
Apple SDKs.
Addresses rdar://158797152
