Reland "[lldb] Refactor variable parsing"

Separates the methods for recursive variable parsing in function
context and non-recursive parsing of global variables.

Original patch: https://reviews.llvm.org/rG601168e42037ac4433e74b920bb22f76d59ba420
Revert patch: https://reviews.llvm.org/rGca5be065c4c612554acdcae3ead01a1474eff296
Diff from the original patch: avoid using nullptr deref/ref.

Differential Revision: https://reviews.llvm.org/D110570