[libTooling] Further simplify `Stencil` type and introduce `MatchComputation`.

Summary:
This revision introduces a new interface `MatchComputation` which generalizes
the `Stencil` interface and replaces the `std::function` interface of
`MatchConsumer`. With this revision, `Stencil` (as an abstraction) becomes just
one collection of implementations of
`MatchComputation<std::string>`. Correspondingly, we remove the `Stencil` class
entirely in favor of a simple type alias, deprecate `MatchConsumer` and change
all functions that accepted `MatchConsumer<std::string>` to use
`MatchComputation<std::string>` instead.

Reviewers: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69802