[ADT] Decouple xxhash.h from ADT. NFC (#196774)
Move xxHash64, xxh3_64bits, and xxh3_128bits ArrayRef/StringRef
overloads from llvm/Support/xxhash.h to inline overloads in
llvm/ADT/ArrayRef.h and llvm/ADT/StringRef.h, so xxhash.h has no ADT
dependencies.
This is prerequisite for using xxh3 as the combine_bytes backend in
llvm/ADT/Hashing.h (#194567), which would otherwise reintroduce a header
dependency cycle.
FoldingSet.h and StableHashing.h adjust to call the new
pointer-and-length entry point.
