This refactors the cstring literal creation as mentioned in the couple of FIXMEs
littered in the various invocations to CreateMetadataVar.  This centralises the
definition of the literals, and will enable changing the literal creation to a
single site.  NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281798 91177308-0d34-0410-b5e6-96231b3b80d8