doxygenfunction fixes

[jakobandersen]

Three fixes:

1. Render the friend keyword in friend functions.
2. Fix (C++) Errors when parsing friend functions in a class #613.
   In the Doxygen XML a friend function does not have function as kind, but friend. Unfortunately so does a friend class. One way to distinguish between them is then to check emptiness of the argsstring.
   E.g., for

   struct D {
           friend class A;
           friend D operator+();
   };

   the xml for the content of D is then

   <memberdef kind="friend" id="structD_1a3c84b3381e0c864a7415d891b6a053ba" prot="public" static="no" const="no" explicit="no" inline="no" virt="non-virtual">
      <type>class</type>
      <definition>friend class A</definition>
      <argsstring></argsstring>
      <name>A</name>
      <param>
         <type><ref refid="structA" kindref="compound">A</ref></type>
      </param>
      ...
   </memberdef>
   <memberdef kind="friend" id="structD_1a35b414831ab33977d59b0e2959c354b4" prot="public" static="no" const="no" explicit="no" inline="no" virt="non-virtual">
      <type><ref refid="structD" kindref="compound">D</ref></type>
       <definition>D operator+</definition>
       <argsstring>()</argsstring>
       <name>operator+</name>
       ...
   </memberdef>

   However, when searching the XML the filters (e.g., https://github.com/michaeljones/breathe/blob/v4.26.0/breathe/renderer/filter.py#L1001) do not search in this XML, but in index.xml which has

   <member refid="structD_1a3c84b3381e0c864a7415d891b6a053ba" kind="friend"><name>A</name></member>
   <member refid="structD_1a35b414831ab33977d59b0e2959c354b4" kind="friend"><name>operator+</name></member>

   and then those index nodes are converted to the corresponding content nodes at some point before they come out at https://github.com/michaeljones/breathe/blob/v4.26.0/breathe/directives.py#L85.
   The filtering of friends is thus done in the directive instead of in the filter.
3. Strengthen check on arguments when there is only one candidate.
   Consider the C++

   void f(int);

   and the rst

   .. doxygenfunction:: f(int, double, std::string)

   This previously succeeded. Now, if parameters are given in the directive, they are always checked for each candidate. The example will now given a lookup warning.

In addition I removed the deduplication of candidates in the lookup warning. I'm not sure when it makes a difference, but it is only in the output, and is more in line with the internal check. Would be good to see an example where duplicates appear.

[vermeeren]

@jakobandersen All good, thanks a lot also for the detailed information in the PR itself.