[libcxxabi/demangle] Remove __cxxabi_config.h include (llvm#101971)

The demangler is shared between libcxxabi and llvm/lib/Demangle, see
libcxxabi/src/demangle/README.txt. The copy in llvm/lib/Demangle cannot
use __cxxabi_config.h. Remove the include. It was only used to identify
clang, which can easily be done without the include as well.

No intended behavior change.

libcxxabi/src/demangle/ItaniumDemangle.h

@@ -19,7 +19,6 @@
 #include "DemangleConfig.h"
 #include "StringViewExtras.h"
 #include "Utility.h"
-#include <__cxxabi_config.h>
 #include <algorithm>
 #include <cctype>
 #include <cstdio>
@@ -31,7 +30,7 @@
 #include <type_traits>
 #include <utility>
 
-#ifdef _LIBCXXABI_COMPILER_CLANG
+#if defined(__clang__)
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wunused-template"
 #endif
@@ -5948,7 +5947,7 @@ struct ManglingParser : AbstractManglingParser<ManglingParser<Alloc>, Alloc> {
 
 DEMANGLE_NAMESPACE_END
 
-#ifdef _LIBCXXABI_COMPILER_CLANG
+#if defined(__clang__)
 #pragma clang diagnostic pop
 #endif