This commit adds a static cast to CONSTEXPR_DCHECK in as_int().

The motivation for this is that currently this check generates the
following compiler warning:

/v8/src/objects/internal-index.h: In member function
‘constexpr int v8::internal::InternalIndex::as_int() const’:
/v8/src/objects/internal-index.h:44:29: warning:
comparison of integer expressions of different signedness:
‘const size_t’ {aka ‘const long unsigned int’} and ‘int’ [-Wsign-compare]
   44 |     CONSTEXPR_DCHECK(entry_ <= std::numeric_limits<int>::max());
      |                      ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Change-Id: I6adda356dbbe522221731a29f205213f1ba23755
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2407892Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69864}