kernel_arg_type_qual metadata that carries constant qualifier is currently untranslatable

To resolve https://github.com/KhronosGroup/SPIRV-LLVM-Translator/issues/942 `NoWrite `SPIR-V attribute was unmapped from `kernel_arg_type_qual` metadata that carries `constant` qualifier (https://github.com/KhronosGroup/SPIRV-LLVM-Translator/pull/989), which left this metadata untranslated. A possible W/A is introduced here: https://github.com/KhronosGroup/SPIRV-LLVM-Translator/pull/996 but it's quiet ugly.

Start by reading issue #942 and the context in PRs #989 and #996. Trace how kernel_arg_type_qual metadata with the constant qualifier is translated, then determine what a clean resolution looks like without the workaround described as ugly in the issue.

Written by the indexing model from the issue text.