[EH] Replace event with tag

[aheejin]

We recently decided to change 'event' to 'tag', and 'event section' to
'tag section', out of the rationale that the section contains a
generalized tag that references a type, which may be used for something
other than exceptions, and the name 'event' can be confusing in the web
context.

See

• Event vs. exception? exception-handling#159 (comment)
• Replace 'event' with 'tag' in explainer exception-handling#161

[aheejin]

Don't know what's going on with clang-tidy... Anyone has a clue? https://github.com/WebAssembly/binaryen/pull/3937/checks?check_run_id=2844692867

[kripken]

No idea... that started happening in every PR that modifies the delegations headers. It's a false positive, but I don't know how to ignore it...

[tlively]

We explicitly #error out in that file if any of the DELEGATE_FIELD_* macros are not defined. In normal usage, we expect those macros to be defined by the including source file. Clang-tidy looks at the header in isolation, though, so it never has a chance to define the macros and our own errors are being triggered.

I haven't seen any configuration options in clang-tidy that could fix this, but if we wanted to get clang-tidy working correctly we could downgrade these #errors to #warnings. We could also do something more complicated like error out if any macros are missing only if at least one is supplied.

[aheejin]

Uploaded #3941 to fix the clang-tidy issue. If that lands, I can rebase this PR onto that.