Documentation cleanup for MacroInfo.

* Clarify what MacroInfo::isBuiltinMacro means, as it really means something
  more like "isMagicalMacro" or "requiresProcessingBeforeExpansion" -- the
  macros defined in "<built-in>" are not considered built-in by this function;
* Escape __LINE__ as \__LINE__ in Doxygen comments so that the underscores
  don't get replaced by *bold* output;
* Turn comments in MacroInfo.cpp into non-Doxygen comments, so that they
  don't result in duplicated/badly formatted Doxygen output;
* Clean up a bunch of \brief formatting, and add a \file comment for
  MacroInfo.h.

llvm-svn: 177581