Comment parsing: refactor handling of command markers in AST

* Use the term 'command marker', because the semantics of 'backslash' and 'at'
  commands are the same.  (Talking about 'at commands' makes them look like a
  special entity.)

* Sink the flag down into bitfields, reducing the size of AST nodes.

* Change the flag into an enum for clarity.  Boolean function parameters are
  not very clear.

* Add unittests for new tok::at_command tokens.

llvm-svn: 176461