Fix pr5470. Tablegen handles template arguments by temporarily setting their

values, resolving references to them, and then removing the definitions.
If a template argument is set to an undefined value, we need to resolve
references to that argument to an explicit undefined value.  The current code
leaves the reference to the template argument as it is, which causes an
assertion failure later when the definition of the template argument is
removed.

llvm-svn: 89581