[Dexter] Mutually exclusive argument group for --builder and --binary

Dexter currently accepts two possible arguments to determine the binary
used for testing; either --builder <builder> (and optionally
compiler/linker flags) to build the binary, or --binary <binary> to use
the provided binary directly. If both are passed, then --binary
overrides --builder; if neither are passed, then an error is raised.
This patch instead puts these arguments into a required mutually
exclusive argument group, so that an error is automatically raised by
argparse if both or neither are given.

As an additional change, the --cflags and --ldflags will now raise a
warning if they are passed without the --builder flag, as they are
meaningless if Dexter is using a pre-built binary.

Reviewed By: Orlando

Differential Revision: https://reviews.llvm.org/D109833