Ignore warnings of glaring uselessness on clang-cl.

I hope those don't fire on regular Windows clang as well, this seems to
be specific to the way how MSVC-compatible template instantiation works.

src/Magnum/ShaderTools/AbstractConverter.h

@@ -1184,12 +1184,22 @@ class MAGNUM_SHADERTOOLS_EXPORT AbstractConverter: public PluginManager::Abstrac
         Containers::Optional<Containers::ArrayView<const char>>(*_inputFileCallback)(const std::string&, InputFileCallbackPolicy, void*){};
         void* _inputFileCallbackUserData{};
 
+        /* clang-cl on Windows complains about this field being unused if the
+           templated setFileCallback() isn't called. Well, sure, it isn't used
+           in that particular case, but what am I supposed to do?! */
+        #ifdef CORRADE_TARGET_CLANG_CL
+        #pragma GCC diagnostic push
+        #pragma GCC diagnostic ignored "-Wunused-private-field"
+        #endif
         /* Used by the templated version only */
         struct FileCallbackTemplate {
             void(*callback)();
             const void* userData;
         /* GCC 4.8 complains loudly about missing initializers otherwise */
         } _inputFileCallbackTemplate{nullptr, nullptr};
+        #ifdef CORRADE_TARGET_CLANG_CL
+        #pragma GCC diagnostic pop
+        #endif
 };
 
 /**

src/Magnum/Text/AbstractFont.h

@@ -805,12 +805,22 @@ class MAGNUM_TEXT_EXPORT AbstractFont: public PluginManager::AbstractPlugin {
         Containers::Optional<Containers::ArrayView<const char>>(*_fileCallback)(const std::string&, InputFileCallbackPolicy, void*){};
         void* _fileCallbackUserData{};
 
+        /* clang-cl on Windows complains about this field being unused if the
+           templated setFileCallback() isn't called. Well, sure, it isn't used
+           in that particular case, but what am I supposed to do?! */
+        #ifdef CORRADE_TARGET_CLANG_CL
+        #pragma GCC diagnostic push
+        #pragma GCC diagnostic ignored "-Wunused-private-field"
+        #endif
         /* Used by the templated version only */
         struct FileCallbackTemplate {
             void(*callback)();
             const void* userData;
         /* GCC 4.8 complains loudly about missing initializers otherwise */
         } _fileCallbackTemplate{nullptr, nullptr};
+        #ifdef CORRADE_TARGET_CLANG_CL
+        #pragma GCC diagnostic pop
+        #endif
 
         Float _size{}, _ascent{}, _descent{}, _lineHeight{};
         UnsignedInt _glyphCount{};

src/Magnum/Trade/AbstractImporter.h

@@ -2614,12 +2614,22 @@ class MAGNUM_TRADE_EXPORT AbstractImporter: public PluginManager::AbstractManagi
         Containers::Optional<Containers::ArrayView<const char>>(*_fileCallback)(const std::string&, InputFileCallbackPolicy, void*){};
         void* _fileCallbackUserData{};
 
+        /* clang-cl on Windows complains about this field being unused if the
+           templated setFileCallback() isn't called. Well, sure, it isn't used
+           in that particular case, but what am I supposed to do?! */
+        #ifdef CORRADE_TARGET_CLANG_CL
+        #pragma GCC diagnostic push
+        #pragma GCC diagnostic ignored "-Wunused-private-field"
+        #endif
         /* Used by the templated version only */
         struct FileCallbackTemplate {
             void(*callback)();
             const void* userData;
         /* GCC 4.8 complains loudly about missing initializers otherwise */
         } _fileCallbackTemplate{nullptr, nullptr};
+        #ifdef CORRADE_TARGET_CLANG_CL
+        #pragma GCC diagnostic pop
+        #endif
 
         #ifdef MAGNUM_BUILD_DEPRECATED
         struct CachedScenes;