[clang] Omit most AttributedStatements from the CFG

`[[clang::fallthrough]]` has meaning for the CFG, but all other
StmtAttrs we currently have don't. So omit them, as AttributedStatements
with children cause several issues and there's no benefit in including
them.

Fixes PR52103 and PR49454. See PR52103 for details.

Differential Revision: https://reviews.llvm.org/D111568