Return of bug#6554 (Compiler error dereferencing multi_array value via an iterator)

The bug described in https://svn.boost.org/trac10/ticket/6554 seems to return. I tried the example from this bug in godbolt: https://godbolt.org/z/4vMf8oo4q and it fails to compile. The code I used is:
```c++
#include

struct data
{
int value;
};
typedef boost::multi_array array_type;

int main() {
array_type a(boost::extents[4][5]);

// ERROR: Cannot compile this line
a.begin()->begin()->value = 7;

// Compiles successfully
(*a.begin()->begin()).value = 5;
return 0;
}
```
All versions of boost on godbolt (1.64 to 1.79) seem to be affected.

Start by reproducing the failing expression from the issue with boost/multi_array.hpp in the linked Godbolt example, then compare it with the dereference form that compiles. Inspect the multi_array iterator path involved in a.begin()->begin()->value. Done means the failing expression compiles and assigns the value as shown, with regression coverage for this example.

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