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.

.clang-tidy

@@ -1,4 +1,4 @@
-Checks:          '-*,clang-diagnostic-*,google-readability-casting,readability-else-after-return,performance-unnecessary-copy-initialization,bugprone-use-after-move,modernize-use-nullptr,modernize-redundant-void-arg,modernize-return-braced-init-list,modernize-use-default-member-init,modernize-use-equals-default,modernize-use-equals-delete,modernize-use-nodiscard,modernize-use-override,cppcoreguidelines-avoid-goto,misc-unconventional-assign-operator,cppcoreguidelines-narrowing-conversions,bugprone-assert-side-effect,bugprone-bool-pointer-implicit-conversion,bugprone-copy-constructor-init,bugprone-forward-declaration-namespace,bugprone-forwarding-reference-overload,bugprone-macro-parentheses'
+Checks:          '-clang-analyzer-security.insecureAPI.bzero,clang-diagnostic-*,google-readability-casting,readability-else-after-return,performance-unnecessary-copy-initialization,bugprone-use-after-move,modernize-use-nullptr,modernize-redundant-void-arg,modernize-return-braced-init-list,modernize-use-default-member-init,modernize-use-equals-default,modernize-use-equals-delete,modernize-use-nodiscard,modernize-use-override,cppcoreguidelines-avoid-goto,misc-unconventional-assign-operator,cppcoreguidelines-narrowing-conversions,bugprone-assert-side-effect,bugprone-bool-pointer-implicit-conversion,bugprone-copy-constructor-init,bugprone-forward-declaration-namespace,bugprone-forwarding-reference-overload,bugprone-macro-parentheses,bugprone-macro-repeated-side-effects,bugprone-move-forwarding-reference,bugprone-misplaced-widening-cast,bugprone-swapped-arguments'
 # It would be nice to enable:
 # - readability-magic-numbers
 # - modernize-avoid-c-arrays

src/ds/bits.h

@@ -42,6 +42,15 @@
 
 #define UNUSED(x) ((void)(x))
 
+#if __has_builtin(__builtin_assume)
+#  define SNMALLOC_ASSUME(x) __builtin_assume(x)
+#else
+#  define SNMALLOC_ASSUME(x) \
+    do \
+    { \
+    } while (0)
+#endif
+
 // #define USE_LZCNT
 
 #include "address.h"

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;
         }