introduce pointer_typet::base_type()

[kroening]

The type of the data that a pointer points to is usually referred to as the
'base type' of the pointer. This commit introduces a method with that name,
to supersede the generic subtype method, whose name is misleading in the
case of pointers.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6580 (4f135cc) into develop (5ff4932) will decrease coverage by 0.00%.
The diff coverage is 79.85%.

@@             Coverage Diff             @@
##           develop    #6580      +/-   ##
===========================================
- Coverage    76.01%   76.01%   -0.01%     
===========================================
  Files         1579     1579              
  Lines       181304   181293      -11     
===========================================
- Hits        137817   137805      -12     
- Misses       43487    43488       +1     

Impacted Files	Coverage Δ
...code/generic_parameter_specialization_map_keys.cpp	100.00% <ø> (ø)
jbmc/src/java_bytecode/goto_check_java.cpp	32.22% <0.00%> (ø)
jbmc/src/java_bytecode/java_types.cpp	94.87% <ø> (ø)
jbmc/src/java_bytecode/java_types.h	96.83% <ø> (ø)
...bytecode_convert_method/convert_invoke_dynamic.cpp	0.00% <0.00%> (ø)
...enerics/parse_generic_class_with_inner_classes.cpp	99.45% <ø> (ø)
src/ansi-c/c_typecheck_expr.cpp	75.02% <0.00%> (ø)
...rc/ansi-c/c_typecheck_gcc_polymorphic_builtins.cpp	76.64% <ø> (ø)
src/goto-instrument/thread_instrumentation.cpp	0.00% <0.00%> (ø)
src/goto-programs/json_expr.cpp	62.20% <0.00%> (ø)
... and 51 more

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