rustllvm: Add LLVMRustArrayType

LLVM internally uses `uint64_t` for array size, but the corresponding
C API (`LLVMArrayType`) uses `unsigned int` so ths value is truncated.
Therefore rustc generates wrong type for fixed-sized large vector e.g.
`[0 x i8]` for `[0u8, ..(1 << 32)]`.

This patch adds `LLVMRustArrayType` function for `uint64_t` support.
auto merge of #14083 : klutzy/rust/llvmrust-arraytype, r=alexcrichton

LLVM internally uses `uint64_t` for array size, but the corresponding
C API (`LLVMArrayType`) uses `unsigned int` so ths value is truncated.
Therefore rustc generates wrong type for fixed-sized large vector e.g.
`[0 x i8]` for `[0u8, ..(1 << 32)]`.

This patch adds `LLVMRustArrayType` function for `uint64_t` support.