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`.

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1929,11 +1929,17 @@ unsafe extern "C" {
         C: &Context,
         effects: MemoryEffects,
     ) -> &Attribute;
+    /// ## Safety
+    /// - Each of `LowerWords` and `UpperWords` must point to an array that is
+    ///   long enough to fully define an integer of size `NumBits`, i.e. each
+    ///   pointer must point to `NumBits.div_ceil(64)` elements or more.
+    /// - The implementation will make its own copy of the pointed-to `u64`
+    ///   values, so the pointers only need to outlive this function call.
     pub(crate) fn LLVMRustCreateRangeAttribute(
         C: &Context,
-        num_bits: c_uint,
-        lower_words: *const u64,
-        upper_words: *const u64,
+        NumBits: c_uint,
+        LowerWords: *const u64,
+        UpperWords: *const u64,
     ) -> &Attribute;
 
     // Operations on functions