clang11: update libc++

[ericLemanissier]

libc++-dev is version 6: https://packages.ubuntu.com/bionic/libc++-dev

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[ericLemanissier]

I just realized it was already proposed in #252 but it looks like there was no newer docker image which came to fix the issue.
Having workarounds like this on CCI is very scary:
https://github.com/conan-io/conan-center-index/blob/1585b34/recipes/imagl/all/conanfile.py#L100-L101
https://github.com/conan-io/conan-center-index/blob/31bc51816892ca56427f9d5550d3890cb91ac6ac/recipes/andreasbuhr-cppcoro/all/conanfile.py#L67-L68
(I'll have to do the same to make conan-io/conan-center-index#7469 pass)

[uilianries]

@ericLemanissier my concern about this PR is backward compatibility. I'm not sure if this change will break or not Conan packages already in Conan center and built by Clang 11 with libc++-6

[ericLemanissier]

according to https://releases.llvm.org/11.1.0/projects/libcxx/docs/DesignDocs/ABIVersioning.html it depends on cmake variable LIBCXX_ABI_VERSION used when building libc++. It is translated into c++ macro _LIBCPP_ABI_VERSION.
Current dockerio/clang11 has: /usr/include/c++/v1/__config:#define _LIBCPP_ABI_VERSION 1
With this PR we get /usr/lib/llvm-11/include/c++/v1/__config:# define _LIBCPP_ABI_VERSION 1 https://dev.azure.com/conanio/conan-docker-tools/_build/results?buildId=908&view=logs&j=e2666bbb-ec41-51a9-9241-02be2ffb2f19&t=9a0fad47-b207-55a8-8ea7-787dc35161c6&l=1889

[uilianries]

I built Poco/1.11.0 and boost/1.76.0@ using this images, with all dependencies using the current libc++ version and worked.