[DebugInfo] Add DIArgList MD to store multple values in DbgVariableIntrinsics

This patch adds a new metadata node, DIArgList, which contains a list of SSA
values. This node is in many ways similar in function to the existing
ValueAsMetadata node, with the difference being that it tracks a list instead of
a single value. Internally, it uses ValueAsMetadata to track the individual
values, but there is also a reasonable amount of DIArgList-specific
value-tracking logic on top of that. Similar to ValueAsMetadata, it is a special
case in parsing and printing due to the fact that it requires a function state
(as it may reference function-local values).

This patch should not result in any immediate functional change; it allows for
DIArgLists to be parsed and printed, but debug variable intrinsics do not yet
recognize them as a valid argument (outside of parsing).

Differential Revision: https://reviews.llvm.org/D88175