[patch] [libcxx] _LIBCPP_WEAK

[G M]

Hi Everyone

The attached patch is for libcxx's new.cpp and __config files. The patch's
intent is to make new.cpp compile using MS's cl.exe compiler without
changing the meaning of anything for any other compiler.

The issue this patch seeks to address is that MS's compiler (cl.exe)
doesn't support the __attribute__((__weak__)) or
__atribute__((__visibility__("default")) syntax; so a solution must be
found where cl.exe doesn't see this syntax.

This patch seeks to solve this problem by changing code patterned like this:
__attribute__((__weak__, __visibility__("default")))
void* operator new(size_t size, const std::nothrow_t&) _NOEXCEPT {
/*snip*/; return p; }

to code like this:
_LIBCPP_WEAK
void* operator new(size_t size, const std::nothrow_t&) _NOEXCEPT { return
p; }

with the expectation that this change will NOT introduce any functionality
change for clang++/g++ etc. That expectation is based on two aspects of the
change:
* The first is the belief that cl.exe doesn't support "weak" in any
documented way and that libcxx on Windows doesn't need it
anyway. So _LIBCPP_WEAK is defined as nothing when cl.exe is the detected
compiler.

For all other compilers, _LIBCPP_WEAK is defined to be just
__attribute__((__weak__)) and nothing more).

This should mean that cl.exe doesn't see the weak attribute syntax and so
won't choke on it; and g++/clang++ will see the same weak attribute that it
saw before this patch.
* The second part is what to do about
__attribute__((_visibility__("default"))) as in the proposed change it is
dropped from the function definition.

The expecatation here is that this is ok because it isn't neccessary
because the prototype for the modified functions already have it; so the
right thing should still happen.
If all of this is correct, then this patch should fix new.cpp for cl.exe
without changing anything else.

It also provides a pattern that will work with all the compilers libcxx
already supports; and without having to introduce alternate #if/#else
guards or other uglyness. This should make it better match the patterns
libcxx already uses.

If removing the "default" attribute turns out to be a problem, I
believe the default attribute could be added back now that it is decoupled
from the "weak" attribute (which I think is a good thing in of itself) by
using one of libcxx's existing macro's such as _LIBCPP_FUNC_VIS /
_LIBCPP_NEW_DELETE_VIS etc.

I'm not sure of the neccessity of LIBCPP_NEW_DELETE_VIS or it's
realtionship to _LIBCPP_FUNC_VIS at this point, FWIW, but that doesn't
matter to the logic of this patch.

I compiled this patch with cl.exe, g++ and clang++.exe.
Please let me know what you think. If this patch doesn't get traction, I'd
appreciate some advice with real alternative code that could be used to
advance things here as I found it hard to produce something actionable from
the comments I received to my previous patch for this problem though I did
and do appreciate the responses.

Thanks
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