Fix building error for error: fallthrough annotation in unreachable code

when building on debug mode on loongarch64/arm64/amd64-linux by clang<14.

Also delete `#ifndef FALLTHROUGH` block as dotnet#98336 added after dotnet#98712.

Author: shushanhf

src/coreclr/interop/comwrappers.cpp

@@ -736,7 +736,9 @@ HRESULT ManagedObjectWrapper::QueryInterface(
                     return E_NOINTERFACE;
 
                 default:
+#if !defined(__clang__) || (__clang_major__ > 13) // Workaround bug in old clang
                     _ASSERTE(false && "Unknown result value");
+#endif
                     FALLTHROUGH;
                 case TryInvokeICustomQueryInterfaceResult::FailedToInvoke:
                     // Set the 'lacks' flag since our attempt to use ICustomQueryInterface

src/coreclr/pal/inc/pal.h

@@ -184,14 +184,6 @@ extern bool g_arm64_atomics_present;
 #define __has_cpp_attribute(x) (0)
 #endif
 
-#ifndef FALLTHROUGH
-#if __has_cpp_attribute(fallthrough)
-#define FALLTHROUGH [[fallthrough]]
-#else // __has_cpp_attribute(fallthrough)
-#define FALLTHROUGH
-#endif // __has_cpp_attribute(fallthrough)
-#endif // FALLTHROUGH
-
 /******************* PAL-Specific Entrypoints *****************************/
 
 #define IsDebuggerPresent PAL_IsDebuggerPresent