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.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177581 91177308-0d34-0410-b5e6-96231b3b80d8