[-Wunsafe-buffer-usage] Fix fixits for span initialized from const size array (#81927)

Example:
int arr[10];
int * ptr = arr;

If ptr is unsafe and we transform it to std::span then the fixit we'd
currently provide transforms the code to:
std::span<int> ptr{arr, 10};

That's suboptimal as that repeats the size of the array in the code.
The idiomatic transformation should rely on the span constructor
that takes just the array argument and relies on template
parameter autodeduction to set the span size.

The transformed code should look like:
std::span<int> ptr = arr;

Note that it just should not change the initializer at all and that also
works for other forms of initialization like:
int * ptr {arr};
becoming:
std::span<int> ptr{arr};

This patch changes the initializer handling to the desired (empty)
fixit.