IR: Introduce GenericDwarfNode

As part of PR22235, introduce `DwarfNode` and `GenericDwarfNode`.  The
former is a metadata node with a DWARF tag.  The latter matches our
current (generic) schema of a header with string (and stringified
integer) data and an arbitrary number of operands.

This doesn't move it into place yet; that change will require a large
number of testcase updates.

llvm-svn: 226529