dump: Add a flag to explicitly disable dumping.
dump: Add a flag to explicitly disable dumping.

While xla_enable_dumping=false is equivalent to the new xla_disable_dumping,
proto3 can't tell the difference between set-to-false explicitly and unset,
and when propagated to the command-line we actually do need to know the
difference. The need is particularly acute when merging stored DebugOptions
with command-line derived DebugOptions, as that merge is done at the proto
level.

As a side effect of this behavior, explicit disabling takes precedence over
explicit enabling.

While here, add a bunch of missing includes/build deps and some basic unit
tests.
