(Towards #2577) Add pointer and target attributes in datatypes, frontend and backend

[JulienRemy]

This starts implementing point 2 of Sergi's list in #2577 by adding new attributes to datatypes, as well as frontend and backend support for them.

Note that is_potentially_aliased is not used anywhere, except to fix an existing Matmul2CodeTrans test.
So this does not check for aliasing anywhere else for now. Stated differently, this might be dangerous to merge as it effectively supports pointers in PSyIR but does not yet restrict transformations to non-aliased variables.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.88%. Comparing base (1e6b095) to head (32d451b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2822      +/-   ##
==========================================
- Coverage   99.88%   99.88%   -0.01%     
==========================================
  Files         357      357              
  Lines       49742    49960     +218     
==========================================
+ Hits        49686    49903     +217     
- Misses         56       57       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.