CodeGen: centralise label construction for method lists

CodeGen: centralise label construction for method lists

Move all the label construction for the various method list emission into
EmitMethodList.  Rather than have all the names be constructed in pieces in all
of the callers of EmitMethodList, have this occur in one site.  This also makes
the calls much easier to understand as we simplify identify the type of the
method list being emitted and the interface name for which it is being emitted.
NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@285023 91177308-0d34-0410-b5e6-96231b3b80d8