[llvm-debuginfo-analyzer] Support both Reference and Type attrs in single DIE

Relax the assumption that at most one Reference-or-Type-like attribute is
present on a DWARF DIE.

Add support for at most one Type attribute (i.e. DW_AT_import xor
DW_AT_type) and separately at most one Reference attribute (i.e.
DW_AT_specification xor DW_AT_abstract_origin xor ...).

Update comment describing old assumption and tag it as a "FIXME" to
reflect the fact that this is perhaps still not general enough.

Add a test based on the case which led me to encounter the bug in the
wild.

Reviewed By: CarlosAlbertoEnciso

Differential Revision: https://reviews.llvm.org/D150713