Reapply "[TableGen] Introduce MatcherList to manage a linked list of Matchers. NFC (#177875)"

With fixes to the slice_after functions to avoid dereferencing the
before_begin() iterator. Deferencing the iterator casts a pointer
to the BeforeBegin object to Matcher*, but the BeforeBegin object
is not a Matcher. This caused a failure when built with gcc.

Original commit message:

The previous memory management involved passing around references to the
std::unique_ptr next pointer in each Matcher. Scopes and
SwitchOpcode/SwitchType did not use std::unique_ptr internally, but
would sometimes need to have the pointers to their children moved to
temporary std::unique_ptrs that may be modified and then put back into
the Scope/Switch. Other code used manual deletion.

This patch introduces a MatcherList object that encapsulates list
management and ownership. The interface is based on std::forward_list
using the libcxx implementation for reference.

Once a Matcher is added to a MatcherList it can only be in that list or
spliced into another list. This ensures it is always owned by a list
that will be responsible for deleting it.

Scope and SwitchOpcode/Type children are now MatcherLists rather than
Matcher*. A nice side effect of this is that we no longer need a Size
field in each Matcher to store the size of Scope/Switch children during
emission. This field was only used for the Matcher that the Scope/Switch
children pointed to. Now we can store it in the MatcherList instead.

There is no change to the generated output.

There's still a little bit of cleanup that can be done in follow ups. For example,
renaming "TheMatcher" and merging FactorScope and FactorNodes together.