Add a small nolint related to #if handling (#6194)

e.g.
https://github.com/carbon-language/carbon-lang/actions/runs/18390546411/job/52399625057?pr=6182

---------

Co-authored-by: josh11b <15258583+josh11b@users.noreply.github.com>

common/raw_hashtable_metadata_group.h

@@ -1001,6 +1001,7 @@ inline auto MetadataGroup::SimdLoad(const uint8_t* metadata, ssize_t index)
   return g;
 }
 
+// NOLINTNEXTLINE(readability-non-const-parameter): Mutation is in #if.
 inline auto MetadataGroup::SimdStore(uint8_t* metadata, ssize_t index) const
     -> void {
 #if CARBON_NEON_SIMD_SUPPORT