[Support] Add llvm::xxh3_128bits

Add a 128-bit xxhash function, following the existing
`llvm::xxh3_64bits` and `llvm::xxHash` implementations.
Previously, 48e93f5 added support for
`llvm::xxh3_64bits`, which closely follows the upstream implementation
at https://github.com/Cyan4973/xxHash, with simplifications from Devin
Hussey's xxhash-clean.
However, it is desirable to have a larger 128-bit hash key for use cases
such as filesystem checksums where chance of collision needs to be
negligible.

So to that end this also ports over `llvm::xxh3_128bits`.

Testing:
- Add a test based on xsum_sanity_check.c in upstream xxhash.

llvm/include/llvm/Support/xxhash.h

@@ -42,13 +42,38 @@
 #include "llvm/ADT/StringRef.h"
 
 namespace llvm {
+
 uint64_t xxHash64(llvm::StringRef Data);
 uint64_t xxHash64(llvm::ArrayRef<uint8_t> Data);
 
 uint64_t xxh3_64bits(ArrayRef<uint8_t> data);
 inline uint64_t xxh3_64bits(StringRef data) {
   return xxh3_64bits(ArrayRef(data.bytes_begin(), data.size()));
 }
-}
+
+/*-**********************************************************************
+ *  XXH3 128-bit variant
+ ************************************************************************/
+
+/*!
+ * @brief The return value from 128-bit hashes.
+ *
+ * Stored in little endian order, although the fields themselves are in native
+ * endianness.
+ */
+struct XXH128_hash_t {
+  uint64_t low64;  /*!< `value & 0xFFFFFFFFFFFFFFFF` */
+  uint64_t high64; /*!< `value >> 64` */
+
+  /// Convenience equality check operator.
+  bool operator==(const XXH128_hash_t rhs) const {
+    return low64 == rhs.low64 && high64 == rhs.high64;
+  }
+};
+
+/// XXH3's 128-bit variant.
+XXH128_hash_t xxh3_128bits(ArrayRef<uint8_t> data);
+
+} // namespace llvm
 
 #endif