Our (TCP) socket support is in a much better state than pipes. Use that
for testing the Communication class.
Move the CreateTCPConnectedSockets function
(SocketTestUtilities.{h,cpp}) to a place where it can be used from
Communication tests.
Just fixing a few things I noticed as I am working on another feature for format
strings in the prompt: forward decls, adding constexpr constructors, various
checks, and unit tests for FormatEntity::Parse and new Definition constructors,
etc.
Reviewed By: teemperor
Differential Revision: https://reviews.llvm.org/D98153