Support opaque pointers

[iliya-diyachkov]

LLVM is switching to opaque pointers and we need to support this in the backend. The general idea is to avoid getPointerElementType() and use getNonOpaquePointerElementType() instead (https://reviews.llvm.org/D116464#inline-1188188). Currenlty we'll convert LLVM’s opaque pointers to SPIRV’s i8* pointers (or to another type, it can be customized) until we have explicit support for opaque pointers in the new SPIRV specification.

Related issue in SPIRV translator's repository KhronosGroup/SPIRV-LLVM-Translator#1444 .