Document config options documentation

[Fly-Style]

Chakra has a lot of useful config options, but it's quite hard to realize if the needed option exists.
My proposition is to create such documentation.

[rhuanjl]

It's a good idea - unfortunately not only is there no separate documentation outside of ConfigFlagsLists.h lots of the stuff in there is defunct so sorting through to find the good ones is awkward.

Helpful flags I make use of:
-mic:x where x is an int - forces CC to jit any given piece of code after it has run x times
-off:simplejit makes CC go straight to the FullJit rather than using the SimpleJit (FullJit has more optimisations hence more things to test for crashes)
-nonative no jit at all
-bgjit- (the - on the end means off rather than on) don't Jit on a background thread, this brings the Jit onto the main process so once jitting starts execution is paused until the jitted code is ready
-trace:bailout - log each to the console whenever a bailout happens, good for catching pathological code patterns AND any bugs to do with bailouts
-dump:bytecode (debug builds only) this outputs the bytecode to the terminal really useful debugging feature
-dump:backend (debug builds only) this outputs the IR from the JIT after each optimisation step to the terminal, not always the easiest to understand BUT if an optimisation is going wrong this is one of the best ways to examine it

[ppenzin]

A few more:

-wasm (on by default) - enable Wasm
-wasmsimd - enable Wasm SIMD
-dump:assembly - dump generated assembly

There were some dead flags - I don't remember which ones though. Depending on how this gets processed we might be able to auto-generate some docs using tools similar to what is intended to be used with API docs.

[rhuanjl]

I'm afraid there is not anything like the header API documentation for these, the flags are defined here and I know quite a few are defunct:
https://github.com/chakra-core/ChakraCore/blob/master/lib/Common/ConfigFlagsList.h

And here are a few basic rules about them:

The things marked PHASE at the top all have 4 related flags:

-off:phase_name //disable this activity - won't work for all phases
-on:phase_name //enable this activity - only relevant for phases that are normally disabled
-trace:phase_name //output a message whenever this activity happens e.g. -trace:bailout logs each bailout to the terminal
-dump:phase_name //debug builds only - dump detailed information about the activity

I think you have to go digging through the codebase (or use trial and error) to work out which options are relevant for each one - whilst there are PHASE_DEFAULT_ON and PHASE_DEFAULT_OFF macros these aren't used consistently e.g. LazyBailout (which is an incomplete feature) is definitely off by default but uses a normal PHASE macro.

Further down the file there are flags, these specify their default values so they're a bit easier to understand, most of these are Booleans which means they're just on or off, these can be used as:

-booleanFlagName //turn on this flag
-booleanFlagName- //turn off this flag

There are also Number and String flags which you can use like:

-numberFlagName:5
-stringFlagName:hi

This PR removes a few of the dead flags: #6564 (as well as a lot of related dead code) There's a lot more dead flags than just these but didn't want to do too much in one go.