Recognize simple current-class objects for unused private method

[kylekatarnls]

Type: bugfix
Issue: Resolves #252 / Resolves #558 / Resolves #937
Breaking change: no

Recognize simple constructions of the current class for unused private method rule, such as:

• parameter typed (with strong type) as an instance of the current class (by name, FQCN, self or static)
• parameter that can be an instance of current class (nullable/union/intersection type that includes the current class)
• new Foo (with Foo matching FQCN of the current class, including self, static)
• clone $this (or clone of a variable recognized as of type of the current class)
• reference to variable that itself matches one of the rule above (recursively)

[codecov]

Codecov Report

Attention: Patch coverage is 59.15493% with 29 lines in your changes are missing coverage. Please review.

Project coverage is 91.88%. Comparing base (2887ff8) to head (8c8a0b6).

❗ Current head 8c8a0b6 differs from pull request most recent head f69e24e. Consider uploading reports for the commit f69e24e to get more accurate results

Files	Patch %	Lines
src/main/php/PHPMD/Rule/UnusedPrivateMethod.php	59.15%	29 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1069      +/-   ##
============================================
- Coverage     92.56%   91.88%   -0.69%     
- Complexity     1252     1286      +34     
============================================
  Files           111      111              
  Lines          3429     3498      +69     
============================================
+ Hits           3174     3214      +40     
- Misses          255      284      +29     

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