Automatically run grammar tests (grammar bot)

[edunham]

How to grammar tests: make check-grammar #2234
How to add bot to Travis build matrix: #28500

[alexcrichton]

cc #15880, I think maybe the original issue? not sure... Has some dependency information though!

[dns2utf8]

I would like to take this on.

[edunham]

@dns2utf8, thanks for volunteering! Is there anything we can do to help you out here?

[fhahn]

@dns2utf8 are you still working on it? If not, I could have a look this weekend.

[dns2utf8]

Sorry it took so long. To answer the questions: The bot needs antlr4, bison, flex and all the build tools required for rustc.

As soon as #37607 is merged, a simple make check-grammar check-lexer will run all current test.

If I did it myself I would probably use a docker container with Arch inside.

[dns2utf8]

I created the PR #37825 but my local dry-runs failed to compile llvm inside any docker container.