[mlir] Align num elements type to LLVM ArrayType (#93230)

MLIR LLMArrayType is using `unsigned` for the number of elements while
LLVM ArrayType is using `uint64_t`
https://github.com/llvm/llvm-project/blob/4ae896fe979b7db501cabde4b6b3504478958682/llvm/include/llvm/IR/DerivedTypes.h#L377

This leads to silent truncation when we use it for globals in flang. 

```
program test
  integer(8), parameter :: large = 2**30
  real,  dimension(large) :: bigarray
  common /c/ bigarray
  bigarray(999) = 666
end
```

The above program would result in a segfault since the global would be
of size 0 because of the silent truncation.

```
fir.global common @c_(dense<0> : vector<4294967296xi8>) : !fir.array<4294967296xi8>
```
became
```
llvm.mlir.global common @c_(dense<0> : vector<4294967296xi8>) {addr_space = 0 : i32} : !llvm.array<0 x i8>
```

This patch updates the definition of MLIR ArrayType to take `uint64_t`
as argument of the number of elements to be compatible with LLVM.