Cleanups 7: Unnecessary guard around new_handler

[StephanTLavavej]

This removes the (very strange) guard #if !defined(_INC_NEW) || !defined(_MSC_EXTENSIONS) around the new_handler typedef.

It appears that this guard may have been intended to avoid duplicate typedefs because <new.h> can include <new> (weirdly). However, duplicate typedefs have always been possible - if <new> was included first, !defined(_INC_NEW) would be true, so <new> would emit new_handler. Then if <new.h> is included, with the default of /Ze, #if defined _MSC_EXTENSIONS && !defined _CORECRT_BUILD would be true, so it would also emit new_handler within namespace std. (See C:\Program Files (x86)\Windows Kits\10\Include\10.0.22000.0\ucrt\new.h.) There's no conflict as long as the typedefs are identical.

With this removal, there is no conflict regardless of inclusion order or /Za:

C:\Temp>type meow.cpp

// clang-format off
#if defined(ONLY_NEW_H)
#include <new.h>
#elif defined(ONLY_STD_NEW)
#include <new>
#elif defined(NEW_H_FIRST)
#include <new.h>
#include <new>
#elif defined(STD_NEW_FIRST)
#include <new>
#include <new.h>
#else
#error WOOF
#endif
// clang-format on
#include <type_traits>
static_assert(std::is_same_v<std::new_handler, void (*)()>, "WOOF");
int main() {}

C:\Temp>cl /EHsc /nologo /W4 /DONLY_NEW_H meow.cpp
meow.cpp

C:\Temp>cl /EHsc /nologo /W4 /DONLY_STD_NEW meow.cpp
meow.cpp

C:\Temp>cl /EHsc /nologo /W4 /DNEW_H_FIRST meow.cpp
meow.cpp

C:\Temp>cl /EHsc /nologo /W4 /DSTD_NEW_FIRST meow.cpp
meow.cpp

C:\Temp>cl /EHsc /nologo /W4 /Za /DONLY_NEW_H meow.cpp
meow.cpp

C:\Temp>cl /EHsc /nologo /W4 /Za /DONLY_STD_NEW meow.cpp
meow.cpp

C:\Temp>cl /EHsc /nologo /W4 /Za /DNEW_H_FIRST meow.cpp
meow.cpp

C:\Temp>cl /EHsc /nologo /W4 /Za /DSTD_NEW_FIRST meow.cpp
meow.cpp

C:\Temp>

This also removes an unnecessary comment (that essentially repeats the type; it's not like the comment alone would teach people how to use this machinery).

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.