Remove LLVM_NODISCARD from StringRef.
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.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@287586 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm