Documentation cleanup: fixing documentation for FrontendAction.
* Fix a typo, s/BeginSourceAction/BeginSourceFile/, so that the documentation
  for FrontendAction::BeginSourceFileAction links correctly to BeginSourceFile;
* Add some basic \file documentation for FrontendAction.h;
* More use of "\brief" instead of repeating the name of the entity being
  documented;
* Stop using Doxygen-style "///" comments in FrontendAction.cpp, as they were
  polluting the documentation for BeginSourceFile;
* Drop incorrect "\see" markup that broke Doxygen's formatting;
* Other minor documentation fixes.
Use the AttributeSet when removing multiple attributes. Use Attribute::AttrKind
when removing one attribute. This further encapsulates the use of the attributes.