c domain: Generate node_id for objects in the right way

[tk0miya]

Feature or Bugfix

• Bugfix
• Refactoring

Purpose

• refs: some domain directives generate wrong target ID #6903

[tk0miya]

I think mention in CHANGES is not needed for C domain. Because it might be not official and hidden feature.

.. c:function:: func

ref with `c.func`_

[tk0miya]

Note: This will conflict with #7256. I'll update either this or #7256 on merging.

[codecov]

Codecov Report

Merging #7269 into 3.x will increase coverage by 0.02%.
The diff coverage is 94.64%.

@@            Coverage Diff             @@
##              3.x    #7269      +/-   ##
==========================================
+ Coverage   84.05%   84.08%   +0.02%     
==========================================
  Files         279      280       +1     
  Lines       43222    43256      +34     
  Branches     6299     6299              
==========================================
+ Hits        36329    36370      +41     
+ Misses       5520     5515       -5     
+ Partials     1373     1371       -2

Impacted Files	Coverage Δ
tests/test_build_html.py	97.91% <ø> (ø)	⬆️
tests/test_domain_c.py	100% <100%> (ø)
sphinx/domains/c.py	75.13% <88%> (+4.16%)	⬆️

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