Introduce a second level of standard substitutions to the mangling,
all of the form `Sc<character>`, and use it to provide standard
substitutions for most of the _Concurrency types.
This is a precursor to rdar://78269642 and a good mangling-size
optimization in its own right.
Previously, they were storing a low-bit flag that indicated that they
were a default actor. Using an extra inhabitant frees up the low bit
for future use without being conspicuously more expensive to check.
