Remove LLVM_NODISCARD from StringRef.

This is a bit too aggressive of a warning, as it is forces
ANY function which returns a StringRef to have its return
value checked.  While useful on classes like llvm::Error which
are designed to require checking, this is not the case for
StringRef, and it is perfectly reasonable to have a function
return a StringRef for which the return value is not checked.

Move LLVM_NODISCARD to each of the individual member functions
where it makes sense instead.

llvm-svn: 287586