[-Wunsafe-buffer-usage] NFC: Introduce an abstraction for fixable code patterns.

This patch introduces a hierarchy of Gadget classes, which are going to be
the core concept in the somewhat sophisticated machine behind the upcoming
-Wunsafe-buffer-usage fix-it hints.

A gadget is a rigid code pattern that constitutes an operation that
the fixit machine "understands" well enough to work with. A gadget may be
"unsafe" (i.e., constitute a raw buffer access) or "safe" (constitute
a use of a pointer or array variable that isn't unsafe per se,
but may need fixing if the participating variable changes type
to a safe container/view).

We'll be able to make decisions about how to fix the code depending on
the set of gadgets that we've enumerated. Basically, in order to fix a type
of a variable, each use of that variable has to participate in a gadget,
and each such gadget has to consent to fixing itself according to
the variable's new type.

Differential Revision: https://reviews.llvm.org/D137348