[modules] Refactor handling of cases where we write an offset to a prior...

[modules] Refactor handling of cases where we write an offset to a prior record into the bitstream and simplify a little, in preparation for doing this in more cases.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@266160 91177308-0d34-0410-b5e6-96231b3b80d8