[lldb] Allow SymbolFileDWARFDebugMap to register multiple compile units

Currently, SymbolFileDWARFDebugMap works on the assumption that there is
only one compile unit per object file. This patch documents this
limitation (when using the general SymbolFile API), and allows users of
the concrete SymbolFileDWARFDebugMap class to find out about these extra
compile units.

Differential Revision: https://reviews.llvm.org/D136114