Simplify equality of address-of-string-constant

[tautschnig]

The back-end assumes that each string is a singleton and, therefore,
taking the address of a particular string constant will always result in
the same value. The simplifier can use this information.

Fixes: #6966

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[codecov]

Codecov Report

Merging #6967 (897034a) into develop (7c89e91) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6967   +/-   ##
========================================
  Coverage    77.83%   77.83%           
========================================
  Files         1568     1568           
  Lines       180361   180365    +4     
========================================
+ Hits        140378   140382    +4     
  Misses       39983    39983           

Impacted Files	Coverage Δ
src/util/simplify_expr_pointer.cpp	85.79% <100.00%> (+0.16%)	⬆️

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[kroening]

We may need to do something about the fact that the C standard doesn't actually guarantee that two identical string literals get the same address.