[c++2a] Implement P0306 __VA_OPT__ (Comma omission and comma deletion)

This patch implements an extension to the preprocessor:

__VA_OPT__(contents) --> which expands into its contents if variadic arguments are supplied to the parent macro, or behaves as an empty token if none.

  - Currently this feature is only enabled for C++2a (this could be enabled, with some careful tweaks, for other dialects with the appropriate extension or compatibility warnings)

  - The patch was reviewed here: https://reviews.llvm.org/D35782 and asides from the above (and moving some of the definition and expansion recognition logic into the corresponding state machines), I believe I incorporated all of Richard's suggestions.

A few technicalities (most of which were clarified through private correspondence between rsmith, hubert and thomas) are worth mentioning.  Given:

    #define F(a,...) a #__VA_OPT__(a ## a)  a ## __VA_OPT__(__VA_ARGS__)

    - The call F(,) Does not supply any tokens for the variadic arguments and hence VA_OPT behaves as a placeholder.
    - When expanding VA_OPT (for e.g. F(,1) token pasting occurs eagerly within its contents if the contents need to be stringified.
    - A hash or a hashhash prior to VA_OPT does not inhibit expansion of arguments if they are the first token within VA_OPT.
    - When a variadic argument is supplied, argument substitution occurs within the contents as does stringification - and these resulting tokens are inserted back into the macro expansions token stream just prior to the entire stream being rescanned and concatenated.

See wg21.link/P0306 for further details on the feature.


Acknowledgment: This patch would have been poorer if not for Richard Smith's usual thoughtful analysis and feedback.
llvm-svn: 315840