[NFC] Re-Core Fingerprint

Switch from a string core to a 128-bit integral core. This should make
Fingerprints much cheaper to copy around and sets us up for a future
where we can provide alternative implementations of the ambient hashing
algorithm.

rdar://72313506

lib/Serialization/ModuleFileCoreTableInfo.h

@@ -600,9 +600,9 @@ public:
   static data_type ReadData(internal_key_type key, const uint8_t *data,
                             unsigned length) {
     using namespace llvm::support;
-    auto str = std::string{reinterpret_cast<const char *>(data),
-                           Fingerprint::DIGEST_LENGTH};
-    return Fingerprint{str};
+    auto str = llvm::StringRef{reinterpret_cast<const char *>(data),
+                               Fingerprint::DIGEST_LENGTH};
+    return Fingerprint::fromString(str);
   }
 };