AK: Add SFINAE fallback for AK C++ concepts use, for Coverity compiler

The Coverity compiler doesn't support C++2a yet, and thus doesn't
even recognize concept keywords. To allow serenity to be built and
analyzed on such compilers, add a fallback underdef to perform
the same template restriction based on AK::EnableIf<..> meta
programming.

Note: Coverity does seem to (annoyingly) define __cpp_concepts, even
though it doesn't support them, so we need to further check for
__COVERITY__ explicitly.

AK/Concepts.h

@@ -30,10 +30,14 @@
 
 namespace AK::Concepts {
 
+#if defined(__cpp_concepts) && !defined(__COVERITY__)
+
 template<typename T>
 concept Integral = IsIntegral<T>::value;
 
 template<typename T>
 concept FloatingPoint = IsFloatingPoint<T>::value;
 
+#endif
+
 }

AK/Stream.h

@@ -62,7 +62,11 @@ public:
     virtual bool discard_or_error(size_t count) = 0;
 };
 
+#if defined(__cpp_concepts) && !defined(__COVERITY__)
 template<Concepts::Integral Integral>
+#else
+template<typename Integral, typename EnableIf<IsIntegral<Integral>::value, int>::Type = 0>
+#endif
 InputStream& operator>>(InputStream& stream, Integral& value)
 {
     stream.read_or_error({ &value, sizeof(value) });
@@ -70,12 +74,18 @@ InputStream& operator>>(InputStream& stream, Integral& value)
 }
 
 #ifndef KERNEL
+
+#if defined(__cpp_concepts) && !defined(__COVERITY__)
 template<Concepts::FloatingPoint FloatingPoint>
+#else
+template<typename FloatingPoint, typename EnableIf<IsFloatingPoint<FloatingPoint>::value, int>::Type = 0>
+#endif
 InputStream& operator>>(InputStream& stream, FloatingPoint& value)
 {
     stream.read_or_error({ &value, sizeof(value) });
     return stream;
 }
+
 #endif
 
 inline InputStream& operator>>(InputStream& stream, bool& value)

AK/Userspace.h

@@ -33,12 +33,12 @@ namespace AK {
 
 // HACK: This is just here to make syntax highlighting work in Qt Creator.
 //       Once it supports C++20 concepts, we can remove this.
-#ifdef __clang__
-template<typename T>
-#else
+#if defined(__cpp_concepts) && !defined(__COVERITY__)
 template<typename T>
 concept PointerTypeName = IsPointer<T>::value;
 template<PointerTypeName T>
+#else
+template<typename T, typename EnableIf<IsPointer<T>::value, int>::Type = 0>
 #endif
 
 class Userspace {