Change EmitRecordWithAbbrevImpl to take Optional record code. NFC.

This change enables EmitRecord to pass the supplied record Code to
EmitRecordWithAbbrevImpl, rather than insert it into the Vals array.
It is an enabler for changing EmitRecord to take an ArrayRef<uintty> instead
of a SmallVectorImpl<uintty>&

Patch suggested by Duncan P. N. Exon Smith, modified by myself a bit to get
correct assertion checking.

llvm-svn: 247186