some better debug-mode parse diagnostics on syntax error

[dsyme]

I worked with @auduchinok today to discuss how to improve parser error recovery

This adds a debug-mode test flag to show the parser stack on parse failure, for use when debugging parser rules
parser stack:

fsc.exe --test:ShowParserStackOnParseError

Also rejigs the internal-use-only flag --ast to show the ast even if parsing has failed, so we can inspect the nodes produced

Typical output of --test:ShowParserStackOnParseError is as follows. The information is limited but actually very useful to understand what's going on. fsprojects/FsLexYacc#138 covers actually printing the full rules but needs an FsLex update

What this shows is that at the point where the syntax error happens, one of four unionCaseReprElements parser rules is still active (this is after an OF token). We got to this state by the states shown from the bottom up (only 12 are shown). That is we were parsing one of these:

tyconDefnRhsBlock  
classDefnBlockKindUnspecified

then tokens arrived that pushed the various further states involving rules for unionTypeRepr and attrUnionCaseDecls and unionCaseReprElements on to the stack.

   ----
       non-terminal NONTERM_unionCaseReprElements: ...
   ----
       non-terminal NONTERM_unionCaseReprElements: ...
       non-terminal NONTERM_unionCaseReprElements: ...
   ----
       non-terminal NONTERM_attrUnionCaseDecl: ...
       non-terminal NONTERM_attrUnionCaseDecl: ...
   ----
       non-terminal NONTERM_attrUnionCaseDecl: ...
       non-terminal NONTERM_attrUnionCaseDecl: ...
       non-terminal NONTERM_attrUnionCaseDecl: ...
       non-terminal NONTERM_attrUnionCaseDecl: ...
       non-terminal NONTERM_attrUnionCaseDecl: ...
   ----
       non-terminal NONTERM_attrUnionCaseDecl: ...
       non-terminal NONTERM_attrUnionCaseDecl: ...
       non-terminal NONTERM_attrUnionCaseDecl: ...
       non-terminal NONTERM_attrUnionCaseDecl: ...
       non-terminal NONTERM_attrUnionCaseDecl: ...
   ----
       non-terminal NONTERM_attrUnionCaseDecl: ...
       non-terminal NONTERM_attrUnionCaseDecl: ...
       non-terminal NONTERM_attrUnionCaseDecl: ...
       non-terminal NONTERM_attrUnionCaseDecl: ...
       non-terminal NONTERM_attrUnionCaseDecl: ...
   ----
       non-terminal NONTERM_attrUnionCaseDecls: ...
   ----
       non-terminal NONTERM_attrUnionCaseDecls: ...
       non-terminal NONTERM_attrUnionCaseDecls: ...
   ----
       non-terminal NONTERM_unionTypeRepr: ...
   ----
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_tyconDefnOrSpfnSimpleRepr: ...
       non-terminal NONTERM_tyconDefnOrSpfnSimpleRepr: ...
       non-terminal NONTERM_tyconDefnOrSpfnSimpleRepr: ...
       non-terminal NONTERM_tyconDefnOrSpfnSimpleRepr: ...
       non-terminal NONTERM_tyconDefnOrSpfnSimpleRepr: ...
   ----
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_classDefnMember: ...
       non-terminal NONTERM_tyconDefnOrSpfnSimpleRepr: ...
       non-terminal NONTERM_tyconDefnOrSpfnSimpleRepr: ...
       non-terminal NONTERM_tyconDefnOrSpfnSimpleRepr: ...
       non-terminal NONTERM_tyconDefnOrSpfnSimpleRepr: ...
       non-terminal NONTERM_tyconDefnOrSpfnSimpleRepr: ...
   ----
       non-terminal NONTERM_tyconDefnRhsBlock: ...
       non-terminal NONTERM_tyconDefnRhsBlock: ...
       non-terminal NONTERM_classDefnBlockKindUnspecified: ...
       non-terminal NONTERM_classDefnBlockKindUnspecified: ...

[KevinRansom]

Looks good