[clang][dataflow] Add `MatchSwitch` utility library.

Adds `MatchSwitch`, a library for simplifying implementation of transfer
functions. `MatchSwitch` supports constructing a "switch" statement, where each
case of the switch is defined by an AST matcher. The cases are considered in
order, like pattern matching in functional languages.

Differential Revision: https://reviews.llvm.org/D120900