Update for several APIs in LLVM that now use StringRefs rather than

const char pointers. In turn, push this through Clang APIs as well,
simplifying a number of bits of code that was handling the oddities of
nullptrs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@246375 91177308-0d34-0410-b5e6-96231b3b80d8