Assert that LLVM range-attribute values don't exceed 128 bits

The underlying implementation of `LLVMCreateConstantRangeAttribute` assumes
that each of `LowerWords` and `UpperWords` points to enough u64 values to
define an integer of the specified bit-length, and will encounter UB if that is
not the case.

Our safe wrapper function always passes pointers to `[u64; 2]` arrays,
regardless of the bit-length specified. That's fine in practice, because scalar
primitives never exceed 128 bits, but it is technically a soundness hole in a
safe function.

We can close the soundness hole by explicitly asserting `size_bits <= 128`.
This is effectively just a stricter version of the existing check that the
value must be small enough to fit in `c_uint`.

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