<rdar://problem/11291668>

Fixed an issue that would happen when using debug map with DWARF in the .o files where we wouldn't ever track down the actual definition for a type when things were in namespaces. We now serialize the decl context information into an intermediate format which allows us to track down the correct definition for a type regardless of which DWARF symbol file it comes from. We do this by creating a "DWARFDeclContext" object that contains the DW_TAG + name for each item in a decl context which we can then use to veto potential accelerator table matches. For example, the accelerator tables store the basename of the type, so if you have "std::vector<int>", we would end up with an accelerator table entry for the type that contained "vector<int>", which we would then search for using a DWARFDeclContext object that contained:

  [0] DW_TAG_class_type "vector<int>"
  [1] DW_TAG_namespace "std"

This is currently used to track down forward declarations for things like "class a::b::Foo;". 

llvm-svn: 155488