Add Clang 13

[uilianries]

Nothing changed between Clang 12 and 13 so far.

The documentation still shows C++17 as default version: https://clang.llvm.org/docs/UsersManual.html#c-language-features

There is no Clang 13 page, because is still in development.

Changelog: Feature: Add Clang 13 support.
Docs: conan-io/docs#2204
closes #9465

/cc @SSE4 @memsharded

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[SSE4]

may you run (perhaps, in docker) the command clang -dM -E -x c++ /dev/null | grep -i __cplusplus just to be sure?

[uilianries]

Should we consider a warning message when using pre-stable versions? It sounds interesting, but unfortunately we could update only on next release, which could be very late after Clang 13 official release. I mean, Docs is cool, but not all people read Docs.

[SSE4]

Should we consider a warning message when using pre-stable versions? It sounds interesting, but unfortunately we could update only on next release, which could be very late after Clang 13 official release. I mean, Docs is cool, but not all people read Docs.

I personally don't think we should.
all these warnings are noisy and may break some CI scripts.
let's not add them, unless really necessary.

[uilianries]

clang -dM -E -x c++ /dev/null | grep -i __cplusplus

@SSE4

# clang --version
Debian clang version 13.0.0-++20210611071718+643b6407faf4-1~exp1~20210611052504.79
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
root@5b5cfc09b6f5:/# clang -dM -E -x c++ /dev/null | grep -i __cplusplus
#define __cplusplus 201402L