doc: expand MSVC-/GCC-specific warning supporessions to clang-cl.

Strangely enough, that C4312 warning suppression now needs to be C4834
instead while I'm pretty sure it was correct back in March 2024 when I
added this in 4f7b57ffd6. C4312 is however
for "conversion of A to B of greater size", and in
4435877cf1 I was fixing a lot of those, so
maybe I just used the same number by accident and because this
particular warning is extremely easy to miss, it was never correct in
the first place? Who knows.

doc/snippets/Math.cpp

@@ -715,12 +715,19 @@ static_cast<void>(d);
 Float sine(Rad angle);
 Float a = 60.0f;
 Deg b;
-/* "warning C4312: discarding return value of function with [[nodiscard]]
+/* "warning C4834: discarding return value of function with [[nodiscard]]
    attribute". Yeah, of course it is. Am I not allowed to write succint code
-   snippets anymore?! */
+   snippets anymore?!
-#ifdef CORRADE_TARGET_MSVC
+
+   Same happens on clang-cl (which uses the MSVC STL), but because clang-cl
+   reports itself as MSVC and for it the MSVC-specific warning suppressions
+   don't work, check for clang-cl first and for MSVC second. */
+#ifdef CORRADE_TARGET_CLANG_CL
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wunused-result"
+#elif defined(CORRADE_TARGET_MSVC)
 #pragma warning(push)
-#pragma warning(disable: 4312)
+#pragma warning(disable: 4834)
 #endif
 /* [Deg-usage-explicit-conversion] */
 //sine(a);                      // compilation error
@@ -731,7 +738,9 @@ std::sin(Float(Rad{b}));        // required explicit conversion hints to user
                                 // that this case needs special attention
                                 // (i.e., conversion to radians)
 /* [Deg-usage-explicit-conversion] */
-#ifdef CORRADE_TARGET_MSVC
+#ifdef CORRADE_TARGET_CLANG_CL
+#pragma GCC diagnostic pop
+#elif defined(CORRADE_TARGET_MSVC)
 #pragma warning(pop)
 #endif
 }

doc/snippets/SceneGraph.cpp

@@ -261,7 +261,8 @@ class MyObject: MyFeature, public Object3D {
 /* [feature-construction-order-crash] */
 }
 {
-#ifdef CORRADE_TARGET_GCC
+/* clang-cl doesn't report itself as GCC but warns too, check it explicitly */
+#if defined(CORRADE_TARGET_GCC) || defined(CORRADE_TARGET_CLANG_CL)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wreorder"
 #endif
@@ -274,7 +275,7 @@ class MyObject: MyFeature, public Object3D {
 };
 /* [feature-construction-order-crash-destruction] */
 }
-#ifdef CORRADE_TARGET_GCC
+#if defined(CORRADE_TARGET_GCC) || defined(CORRADE_TARGET_CLANG_CL)
 #pragma GCC diagnostic pop
 #endif
 }