llvm-cvtres: Split addChild(ID) into two functions

Before, there was an IsData parameter. Now, there are two different
functions for data nodes and ID nodes. No behavior change, needed for a
follow-up change to make two data nodes (but not two ID nodes) with the
same ID an error.

For consistency, rename another addChild() overload to addNameChild().

llvm-svn: 359024