[C++] Rare bad buffer content alignment if sizeof(T) != alignof(T) (#7520)

* [C++] Add a failing unit test for #7516 (Rare bad buffer content alignment if sizeof(T) != alignof(T))

* [C++] Fix final buffer alignment when using an array of structs
* A struct can have an arbitrary size and therefore sizeof(struct) == alignof(struct)
  does not hold anymore as for value primitives.
* This patch fixes this by introducing alignment parameters to various
  CreateVector*/StartVector calls.
* Closes #7516

tests/alignment_test.cpp

@@ -0,0 +1,31 @@
+#include "alignment_test.h"
+
+#include "flatbuffers/flatbuffer_builder.h"
+#include "alignment_test_generated.h"
+#include "test_assert.h"
+
+namespace flatbuffers {
+namespace tests {
+
+void AlignmentTest() {
+  FlatBufferBuilder builder;
+
+  BadAlignmentLarge large;
+  Offset<OuterLarge> outer_large = CreateOuterLarge(builder, &large);
+
+  BadAlignmentSmall *small;
+  Offset<Vector<const BadAlignmentSmall *>> small_offset =
+      builder.CreateUninitializedVectorOfStructs(9, &small);
+  (void)small; // We do not have to write data to trigger the test failure
+
+  Offset<BadAlignmentRoot> root =
+      CreateBadAlignmentRoot(builder, outer_large, small_offset);
+
+  builder.Finish(root);
+
+  Verifier verifier(builder.GetBufferPointer(), builder.GetSize());
+  TEST_ASSERT(VerifyBadAlignmentRootBuffer(verifier));
+}
+
+}  // namespace tests
+}  // namespace flatbuffers