Update to reflect changes in Clang.

Clang r175978 changed PPCallbacks to take MacroDirective instead of MacroInfo.

git-svn-id: http://include-what-you-use.googlecode.com/svn/trunk@449 812d1f90-f944-d88a-411a-122bdc93e1ea
Update comment referring to renamed parameter.

git-svn-id: http://include-what-you-use.googlecode.com/svn/trunk@450 812d1f90-f944-d88a-411a-122bdc93e1ea