Parse C++ class and inheritance from Debug Info

[ttreyer]

Extend the Dwarf parser to make class members and parents' members accessible.

This patch handles any shape of inheritance hierarchy, but does provide access to parents' shadowed members:

struct Parent { int abc, xyz; };
struct Child : Parent { int abc; };
...
child->abc          // Access the child's member
child->xyz          // Access the parent's member
child->Parent::abc  // Not implemented: access the parent's shadowed member

Multiple inheritance suffer from a similar problem: when a parent shadows the member of another parent.

struct GrandChild : Parent, Child {};
g_child->xyz // Access Parent's member
g_child->abc // Ambiguous! Will access Parent::abc, but it should reject and require a qualified name instead

Checklist

• Language changes are updated in man/adoc/bpftrace.adoc
• User-visible and non-trivial changes updated in CHANGELOG.md
• The new behaviour is covered by tests