Enable the static analyser from clang-tidy.

Fix a false positive.  The loops in the pagemap accessor are too
complicated for the static analyser to check, so it doesn't spot that a
non-null return is impossible.

src/mem/pagemap.h

@@ -281,6 +281,7 @@ namespace snmalloc
 
         for (; ix < last; ix++)
         {
+          SNMALLOC_ASSUME(leaf_ix.first != nullptr);
           leaf_ix.first->values[ix] = x;
         }