Strange behavior when mixing -fexceptions with -fwasm-exceptions

We have a nightmare build system that uses a mix of cmake and direct emcc invocations to first compile a bunch of static libraries, then link them together into a wasm file in a configurable way. I'm working on utilizing `EXPORT_EXCEPTION_HANDLING_HELPERS` so that our JIT can interop with the code emscripten produces for C++ exception handling. I have been getting lots of baffling failures in the process, and I think I finally tracked the problem down to this:

The configuration parser sets internal flags when parsing `-fno-exceptions`, `-fexceptions`, `-fignore-exceptions` and `-fwasm-exceptions`. However, each of those flags does not set all of the internal flags, only some of them. Our build system had some `-fexceptions` flags buried deep in the configuration, so when I added `-fwasm-exceptions` I got a weird unique configuration different from if I only had set the latter. (This was hard to diagnose because it seems like cmake and/or emcmake delete the .json file passed to the compiler upon build failure, and i couldn't get `-v` to flow through to the compiler either, so I couldn't inspect the compiler arguments...)

See https://github.com/emscripten-core/emscripten/blob/c47ab8d7f644c6a501a0fc0eeb91f60c5b6682cb/emcc.py#L3610-L3627

Based on my reading of this, the configuration `-fexceptions -fwasm-exceptions` would result in the internal flags `{ WASM_EXCEPTIONS: 1, DISABLE_EXCEPTION_THROWING: 0, DISABLE_EXCEPTION_CATCHING: 0 }` while `-fwasm-exceptions` would only produce `{ WASM_EXCEPTIONS: 1 }`. Depending on parsing order some of the other ones could also produce a weird configuration and it seems like using mutually exclusive flags would not spit out any sort of error or warning, you'd just get a weird compile configuration.

In the process of troubleshooting everything I was getting confusing errors like `emcc : error : DISABLE_EXCEPTION_CATCHING=0 is not compatible with -fwasm-exceptions` (why would disabling catching be mandatory if I enabled EH? it turns out this switch means something other than what its name implies) and `EXPORT_EXCEPTION_HANDLING_HELPERS requires either of -fexceptions or -fwasm-exceptions` even though I was definitely passing through `-fwasm-exceptions`. I also saw `DISABLE_EXCEPTION_THROWING was set (probably from -fno-exceptions)` a lot even though I was trying to enable exceptions, presumably because some other flag was overwriting `-fwasm-exceptions`.

I don't have a reliable repro for this because I was never able to actually capture the .rsp or .json files due to some part of the emcmake pipeline deleting them :( Just wanted to describe this situation I ran into and suggest that maybe the argument/switch parsing for this set of flags be cleaned up so that incoherent combinations of switches produce a warning or error.

My suggestion would be that `-fno-exceptions`, `-fexceptions`, and `-fwasm-exceptions` should be mutually exclusive, or at least produce a warning if you use more than one of them, because setting two or more of them is not equivalent to only setting the last one. Including `-fignore-exceptions` in the list would complicate things further so maybe it should be required to only come after the other three have been set.