The bcc rewriter currently traces external pointers using
ProbeVisitor in order to replace dereferences with calls to
bpf_probe_read. It is, however, unable to trace them through maps.

This commit remedy this for simple (yet common) cases. Through a
first traversal of the Clang AST, MapVisitor looks for calls to
update (and insert) to find maps with an external pointer as value.
When ProbeVisitor runs, in a second time, it looks for calls to
lookup (and lookup_or_init). If the map was registered as having an
external pointer as value, the l-value of the lookup assignment is
marked as being an external pointer.

Two traversals of the Clang AST are needed because the update of a
map may happen after the lookup in the AST. Therefore, the first
traversal makes sure we inspect all updates before acting on lookups.
To implement this two-stage traversal without parsing the AST twice,
ProbeConsumer and BTypeConsumer now implement HandleTranslationUnit,
which is called after a whole translation unit has been parsed.