When importing from LLVM IR the data layout of all pointer types
contains an index bitwidth that should be used for index computations.
This revision adds a getter to the DataLayout that provides access to
the already stored bitwidth. The function returns an optional since only
pointer-like types have an index bitwidth. Querying the bitwidth of a
non-pointer type returns std::nullopt.
The new function works for the built-in Index type and, using a type
interface, for the LLVMPointerType.
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