change mangling of pointers

[simeonschaub]

This follows the mangling scheme as described in https://www.khronos.org/files/opencl-spir-12-provisional.pdf (page 25)

The Khronos translator seems to like this mangling better, see KhronosGroup/SPIRV-LLVM-Translator#3404

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.19%. Comparing base (d6c49fb) to head (96326e5).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #395   +/-   ##
=======================================
  Coverage   80.19%   80.19%           
=======================================
  Files          12       12           
  Lines         722      722           
=======================================
  Hits          579      579           
  Misses        143      143           

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