BitcodeWriter: Simplify and test writing blobs, NFC

Split helper out of EmitRecordWithAbbrevImpl called emitBlob to reduce
code duplication, and add a few tests for it.

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264550 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed