-fsanitize=function,MicrosoftMangle: Switch to xxh3_64bits

Following recent changes switching from xxh64 to xxh32 for better
hashing performance (e.g., D154813). These particular instances likely
have negligible time, but this change moves us toward removing xxHash64.

The type hash for -fsanitize=function will change, following a recent
change D148785 (not in any release yet) to the type hash scheme, though
sanitizers don't sign up for cross-version compatibility anyway.

The MicrosoftMangle instance is for internal symbols that need no
compatibility guarantee, as emphasized by the comment.