Until now all debug info MDNodes referred to a root MDNode, a compile unit....

Until now all debug info MDNodes referred to a root MDNode, a compile unit. This simplified handling of these needs in dwarf writer.  However, one side effect of this is that during link time optimization all these MDNodes are _not_ uniqued. In other words there will be N number of MDNodes describing "int", "char" and all other types, which would suddenly grow when each object file starts using libraries like STL.

MDNodes graph structure such that compiler unit keeps track of important MDNodes and update dwarf writer to process mdnodes top-down instead of bottom up.

llvm-svn: 137778