Debug Info: Fix LTO type uniquing for C++ member declarations

based on the ODR.

This adds an OdrMemberMap to DwarfDebug which is used to unique C++
member function declarations based on the unique identifier of their
containing class and their mangled name.
We can't use the usual DIRef mechanism here because DIScopes are indexed
using their entire MDNode, including decl_file and decl_line, which need
not be unique (see testcase).

Prior to this change multiple redundant member function declarations would
end up in the same uniqued DW_TAG_class_type.

llvm-svn: 203982