clc: Fix const arg metadata detection
This is copied from !13449 but into the other CL metadata parsing code. This fixes 2 things:
- For LLVM 13, the translator no longer emits
kernel_arg_type_qualstring to find it too.
- The CL API expects args in the
__constantaddress space to also be marked const. CLOn12 has logic for this, but go ahead and do it in the metadata parsing so the compiler output is the same between new and old LLVM.