Add two missing entries to the C++11 support page. Bump one relevant diagnostic
(for an integer too large for any signed type) from Warning to ExtWarn -- it's
ill-formed in C++11 and C99 onwards, and UB during translation in C89 and
C++98. Add diagnostic groups for two relevant diagnostics.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203974 91177308-0d34-0410-b5e6-96231b3b80d8