Only a subset of the fields of DbgVariable are meaningful at any time,
and some fields are re-used for multiple purposes (for example
FrameIndexExprs is used with a throw-away frame-index of 0 to hold a
single DIExpression without needing to add another member). The exact
invariants must be reverse-engineered by inspecting the actual use of
the class, its imprecise/outdated doc-comment, and some asserts.
Refactor DbgVariable into a sum type by inheriting from std::variant.
This makes the active fields for any given state explicit and removes
the need to re-use fields in disparate contexts. As a bonus, it seems to
reduce the size on my x86_64 linux box from 144 bytes to 96 bytes.
There is some potential cost to std::get as it must check the active
alternative even when context or an assert obviates it. To try to help
ensure the compiler can optimize out the checks the patch also adds a
helper get method which uses the noexcept std::get_if.
Some of the extra cost would also be avoided more cleanly with a
refactor that exposes the alternative types in the public interface,
which will come in another patch.