[clang][dataflow] Allow analyzing multiple functions in unit tests

In unit tests for concrete dataflow analyses we typically use the
testonly `checkDataflow()` helper to analyse a free function called
"target". This pattern allows our tests to be uniform and focused on
specific statement- or expression-level C++ features.

As we expand our feature coverage, we want to analyze functions whose
names we don't fully control, like constructors, destructors, operators
etc. In such tests it is often convenient to analyze all functions
defined in the input code, to avoid having to carefully craft an AST
matcher that finds the exact function we're interested in. That can be
easily done by providing `checkDataflow()` with a catch-all matcher like
`functionDecl()`.

It is also often convenient to define multiple special member functions
in a single unit test, for example, multiple constructors, and share the
rest of the class definition code between constructors. As a result, it
makes sense to analyze multiple functions in one unit test.

This change allows `checkDataflow()` to correctly handle AST matchers
that match more than one function. Previously, it would only ever
analyze the first matched function, and silently ignore the rest. Now it
runs dataflow analysis in a loop, and calls `VerifyResults` for each
function that was found in the input and analyzed.

Reviewed By: ymandel, sgatev

Differential Revision: https://reviews.llvm.org/D140859