[clang][dataflow] Allow MatchSwitch to return a value

This patch adds another `typename` parameter to `MatchSwitch` class: `Result` (defaults to `void`), corresponding to the return type of the function. This necessitates a couple minor changes to the `MatchSwitchBuilder` class, and is tested via a new `ReturnNonVoid` test in `clang/unittests/Analysis/FlowSensitive/MatchSwitchTest.cpp`.

Reviewed By: gribozavr2, sgatev, xazax.hun

Differential Revision: https://reviews.llvm.org/D128467