Separated the DWARF index for types from that the index of the namespaces

since we can't parse DW_TAG_namespace DIEs as types. They are only decls in
clang. All of the types we handle right now have both clang "XXXType" classes
to go with the "XXXDecl" classes which means they can be used within the 
lldb_private::Type class. I need to check to see which other decls that don't
have associated type objects need to float around the debugger and possibly
make a lldb_private::Decl class to manage them.

llvm-svn: 116558