[lldb] Allow dumping the state of all scratch TypeSystems

This adds the `target dump typesystem'`command which dumps the TypeSystem of the
target itself (aka the 'scratch TypeSystem'). This is similar to `target modules
dump ast` which dumps the AST of lldb::Modules associated with a selected
target.

Unlike `target modules dump ast`, the new command is not a subcommand of `target
modules dump` as it's not touching the modules of a target at all. Also unlike
`target modules dump ast` I tried to keep the implementation language-neutral,
so this patch moves our Clang `Dump` to the `TypeSystem` interface so it will
also dump the state of any future/downstream scratch TypeSystems (e.g., Swift).
That's also why the command just refers to a 'typesystem' instead of an 'ast'
(which is only how Clang is necessarily modelling the internal TypeSystem
state).

The main motivation for this patch is that I need to write some tests that check
for duplicates in the ScratchTypeSystemClang of a target. There is currently no
way to check for this at the moment (beside measuring memory consumption of
course). It's probably also useful for debugging LLDB itself.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D111936