coroutines: Fix whitespace and comment markers.

This amends the cases where inline comments in function calls were
followed by a space.  It also fixes some uses of C++ style and wrongly
wrapped comment end markers.

gcc/cp/ChangeLog:

2020-01-26  Iain Sandoe  <iain@sandoe.co.uk>

* coroutines.cc: Amend whitespace after inline comments
throughout.  Ensure use of C-style comment markers.