[upstream-with-swift] Temporary workaround for LLVM r334283
[upstream-with-swift] Temporary workaround for LLVM r334283

LLVM r334283 changed StringRef.h so that split(char) and rsplit(char) are
implemented using [r]split(StringRef), which does not have an inlinable
definition in the header file. Because the Swift runtime and other target
libraries use StringRef and other ADT headers without actually linking
libSupport, this causes link failures. Changing to build libSupport for
every target is complicated, so we may just need to tweak Swift to avoid
using these StringRef functions. This change just puts back the old inline
implementations until we can sort that out.

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm