[libTooling] Simplify type structure of `Stencil`s.

Summary:
Currently, stencils are defined as a sequence of `StencilParts`. This
differentiation adds an unneeded layer of complexity to the definition of
Stencils. This change significantly simplifies the type structure: a stencil is
now conceptually any object implementing `StencilInterface` and `Stencil` is
just a thin wrapper for pointers to this interface.

To account for the sequencing that was supported by the old `Stencil` type, we
introduce a sequencing class that implements `StencilInterface`. That is,
sequences are just another kind of Stencil and no longer have any special
status.

Corresponding to this change in the type structure, we change the way `cat` is
used (and defined). `cat` bundles multiple features: it builds a stencil from a
sequence of subcomponents and admits multiple different types for its arguments,
while coercing them into the right type. Previously, `cat` was also used to
coerce a single `StencilPart` into a `Stencil`. With that distinction gone, many
uses of `cat` (e.g. in the tests) are unnecessary and have, therefore, been
removed.

Reviewers: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69613