libclang: don't store nul-terminated strings as StringRefs,
if the nul-terminatedness property is important for clients.

Also, don't return the same CXString multiple times.  This did not create a
correctness issue in practice because the CXString was of an CXS_Unmanaged
kind, and destruction was a no-op.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175455 91177308-0d34-0410-b5e6-96231b3b80d8