[libc++] Fix test for vector data_const.pass.cpp (#122085)

The test contained some non-const accesses when we're really trying to test
that the method is const.

Author: 2LoS

libcxx/test/std/containers/sequences/vector/vector.data/data_const.pass.cpp

@@ -39,7 +39,7 @@ TEST_CONSTEXPR_CXX20 bool tests()
         assert(is_contiguous_container_asan_correct(v));
     }
     {
-        std::vector<Nasty> v(100);
+        const std::vector<Nasty> v(100);
         assert(v.data() == std::addressof(v.front()));
         assert(is_contiguous_container_asan_correct(v));
     }
@@ -55,7 +55,7 @@ TEST_CONSTEXPR_CXX20 bool tests()
         assert(is_contiguous_container_asan_correct(v));
     }
     {
-        std::vector<Nasty, min_allocator<Nasty>> v(100);
+        const std::vector<Nasty, min_allocator<Nasty>> v(100);
         assert(v.data() == std::addressof(v.front()));
         assert(is_contiguous_container_asan_correct(v));
     }
@@ -70,7 +70,7 @@ TEST_CONSTEXPR_CXX20 bool tests()
       assert(is_contiguous_container_asan_correct(v));
     }
     {
-      std::vector<Nasty, safe_allocator<Nasty>> v(100);
+      const std::vector<Nasty, safe_allocator<Nasty>> v(100);
       assert(v.data() == std::addressof(v.front()));
       assert(is_contiguous_container_asan_correct(v));
     }