enabled and mitigated `readability-const-return-type` clang-tidy warnings (#5644)

.clang-tidy

@@ -59,7 +59,6 @@ Checks: >
         -portability-std-allocator-const,
         -readability-avoid-const-params-in-decls,
         -readability-braces-around-statements,
-        -readability-const-return-type,
         -readability-container-data-pointer,
         -readability-function-cognitive-complexity,
         -readability-function-size,

clang-tidy.md

@@ -118,7 +118,6 @@ Also reports a false positive about templates which deduce the array length: htt
 We run this separately via `clang-include-cleaner` in the `iwyu.yml` workflow as the findings of the include checkers still need to be reviewed manually before applying them. 
 
 `bugprone-branch-clone`<br/>
-`readability-const-return-type`<br/>
 `modernize-return-braced-init-list`<br/>
 `misc-throw-by-value-catch-by-reference`<br/>
 `readability-avoid-const-params-in-decls`<br/>

gui/test/translationhandler/testtranslationhandler.cpp

@@ -24,7 +24,7 @@
 #include <QStringList>
 #include <QtTest>
 
-static const QStringList getTranslationNames(const TranslationHandler& handler)
+static QStringList getTranslationNames(const TranslationHandler& handler)
 {
     QStringList names;
     for (const TranslationInfo& translation : handler.getTranslations()) {

lib/standards.h

@@ -59,7 +59,7 @@ struct Standards {
         }
         return false;
     }
-    const std::string getC() const {
+    std::string getC() const {
         switch (c) {
         case C89:
             return "c89";

lib/utils.h

@@ -36,6 +36,7 @@
 
 struct SelectMapKeys {
     template<class Pair>
+    // NOLINTNEXTLINE(readability-const-return-type) - false positive
     typename Pair::first_type operator()(const Pair& p) const {
         return p.first;
     }