[libcxx] r286789 - Add check-cxx-abilist target when supported.

[Justin Bogner via cfe-commits]

Yep, the copy-libcxx-headers-into-build-tree logic is so that they end
up somewhere where a just built clang will find them relative to itself
(it looks for something like ../include/c++/v1). It's purely a
convenience so you don't need to call `ninja install` to use a clang
that refers to headers that match your checked out libc++.

Chris Bieneman via cfe-commits <cfe-commits at lists.llvm.org> writes:
> (+Bogner, I think he originally added this)
>
> My understanding is that we only did this copy step if you are
> building with a clang that isn't installed so that the non-installed
> clang could find these headers.
>
> I'm pretty sure compiler-rt finds libcxx via the sources, not the
> build directory.
>
> I'll put together a fix later today.
>
> -Chris
>
>> On Dec 14, 2016, at 11:49 PM, Eric Fiselier <eric at efcs.ca> wrote:
>> 
>>  
>> Eric, this part of this change seems wrong to me. It causes the
>> headers to be installed into the libcxx build directory instead of
>> the LLVM one. If you build using the LLVM runtimes directory this
>> puts the headers in the wrong place for clang to find them.
>> 
>> Is there a reason you're copying them into the libcxx binary dir? I
>> generally don't think the headers are needed there.
>> 
>> 
>> If it seems wrong for building in the LLVM runtimes directory then
>> it probably is. I don't think there is a good reason why we need to
>> copy the headers into the build directory, although we should double
>> check compiler-rt uses the headers from the source directory.
>> 
>> If you know how to fix this feel free to commit a patch. If not I'll
>> look into this more on Friday.
>> 
>> @Dan, @Jonathan: Do you guys use, or see a need for, this behavior?
>> 
>
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