Function hashmix

src/ast/ast_typedecl.cpp:473–480

    // semantic hash

    __forceinline uint64_t hashmix ( uint64_t hash, uint64_t key ) {
        hash ^= key;
        hash *= UINT64_C(0x9e3779b97f4a7c15);
        hash ^= rotr64_c(hash,31);
        hash *= UINT64_C(0x9e3779b97f4a7c15);
        hash ^= rotr64_c(hash, 28);
        return hash;
    }

    // Save/clear/restore the lazy findAlias cache bits (PR #2849) so they don't
    // enter the lookup/semantic hash. The bits reflect runtime cache state, not