build: lint header files with clang-tidy (#7527)

* build: clang-tidy headers when building libtransmission

* chore: revert `= default` workaround

It was introduced in 6909ec0bad to fix build issues with macOS 10.14. We
no longer support that version.

* fix: clang-tidy warnings for libtransmission

* build: clang-tidy headers when building tests

* fix: clang-tidy warnings for tests

* build: clang-tidy headers when building qt

* code review: don't manually edit mime-types.h

* code review: unify variable naming for static private members

libtransmission/error.h

@@ -37,7 +37,7 @@ public:
         return code_ != 0;
     }
 
-    [[nodiscard]] constexpr operator bool() const noexcept
+    [[nodiscard]] explicit constexpr operator bool() const noexcept
     {
         return has_value();
     }