Adjust the StringProcessing module to follow the defaults of SwiftCore
with regards to generic metadata prespecialization and the library
evolution modes.
Update the `find_package` to mark `SwiftCore` as `REQUIRED`. This also
re-orders some of the declarations to make the CMakeLists.txt layout
more uniform across the projects.
Extract the `install_swift_interface` function into a new file. This
is always invoked to perform the installation of the Swift interfaces as
well as the binary module. Additionally, it prepares for a future
refactoring which will also install the swift documentation interface.
Introduce install targets for the StringProcessing module. This is a
prerequisite for building a static variant of the standard library to
enable the statically linked early swift driver for Windows to
bootstrap.
