String version of BuildMul(...) calls itself instead of span overload

[TillAlex]

Calling the string version of BuildMul leads to a stack overflow because it calls itself instead of span version:

https://github.com/microsoft/LLVMSharp/blob/101774800b7f03b7a492f882adacbe1d5fcc0499/sources/LLVMSharp/Interop.Extensions/LLVMBuilderRef.cs#L426

[tannergooding]

Oops, will get a fix up later tonight if no one else grabs it first.