Fix build with clang 16

clang 16 reports a missing declaration in new-op.cc.  We believed
these operators to be declared starting with C++14, but apparently
that is not the case.

This patch reverts the earlier change and then updates the comment to
reflect the current state.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31141

gdbsupport/new-op.cc

@@ -27,6 +27,11 @@
 #include "host-defs.h"
 #include <new>
 
+/* These are declared in <new> starting C++14, but removing them
+   caused a build failure with clang.  See PR build/31141.  */
+extern void operator delete (void *p, std::size_t) noexcept;
+extern void operator delete[] (void *p, std::size_t) noexcept;
+
 /* Override operator new / operator new[], in order to internal_error
    on allocation failure and thus query the user for abort/core
    dump/continue, just like xmalloc does.  We don't do this from a