Feedback on clang change (#53194)

[Ken Cunningham]

>what's the benefit of having clang compile software against gcc's libstdc++ vs. gcc6
compiling it.

The only benefit is that clang handles objc and macos SDK-heavy ports
when gcc6 will not.

>And what's the relation of all this to the magic
-D_GLIBCXX_USE_CXX11_ABI=0 flag.

This flag lets libgcc's stdlibc++ speak the same "c++ dialect" as the
built-in system stdlibc++, making the built libraries and executables
compatible with each other

>have this enabled for all versions of clang

Only clang-3.9 or newer can do the libgcc trick. It will not be ported
to clang-3.7 or clang-3.8.

> And we should finally address binary builds for libc++.

I agree. The buildbot issue is driving this.

I addition to the buildbot problem, there are two other reasons why
this libgcc stdlibc++ thing might be interesting.
1. xcode built ports will always build against stdlibc++, and fixing
them to build against libc++ is hard.

2. libgcc's stdlibc++ allows thread-local storage on < 10.9, which
allows some ports to build that will not build against libc++