[libclang] Have clang_getCursorSpelling() return the string for a CXCursor_ObjCStringLiteral or CXCursor_StringLiteral cursor.

rdar://16206459

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202745 91177308-0d34-0410-b5e6-96231b3b80d8