BitcodeReader: Allow METADATA_STRINGS to only have !""

Support parsing a METADATA_STRINGS record that only has a single piece
of metadata, !"".  Fixes a corner case in r264551.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264699 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed