Nominate Shafik Yaghmour and Vlad Serebrennikov for C++ conformance

Shafik and Vlad are both members of WG21 and both have familiarity with
reasoning about the C++ standard. They've both volunteered to help
answer conformance related questions, and this is an area where we get
quite a bit of questions so having a larger stable of maintainers is
quite useful.

Author: AaronBallman

clang/Maintainers.rst

@@ -231,6 +231,12 @@ C++ conformance
 | Hubert Tong
 | hubert.reinterpretcast\@gmail.com (email), hubert.reinterpretcast (Phabricator), hubert-reinterpretcast (GitHub)
 
+| Shafik Yaghmour
+| shafik.yaghmour\@intel.com (email), shafik (GitHub), shafik.yaghmour (Discord), shafik (Discourse)
+
+| Vlad Serebrennikov
+| serebrennikov.vladislav\@gmail.com (email), Endilll (GitHub), Endill (Discord), Endill (Discourse)
+
 
 C++ Defect Reports
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