Remove SymPair and instead use two DefinedRegulars instead.

I noticed that I was looking for the definition of SymPair when hacking
the Writer, only to find that it is just a pair of DefinedRegular symbols.
I don't think it provides more values than the cost of using brainpower
to memorize the type. I didn't roll back r266317, which introduced SymPair,
because the patch removes code repetitions. I ported that change to new
code.

llvm-svn: 267047