DWARF: Implement DW_AT_signature lookup for type unit support
This patch implements the main feature of type units. When completing a
type, if we encounter a DW_AT_signature attribute, we use it's value to
lookup the complete definition of the type in the relevant type unit.
To enable this lookup, we build up a map of all type units in a symbol
file when parsing the units. Then we consult this map when resolving the
I include add a couple of tests which exercise the type lookup feature,
including one that ensure we do something reasonable in case we fail to
lookup the type.
Reviewers: clayborg, JDevlieghere, aprantl, alexshap
Subscribers: mgrang, lldb-commits
Differential Revision: https://reviews.llvm.org/D62246