For each macro directive (define, undefine, visibility) have a separate object that gets chained
to the macro directive history. This has several benefits:
-No need to mutate a MacroDirective when there is a undefine/visibility directive. Stuff like
PPMutationListener become unnecessary.
-No need to keep extra source locations for the undef/visibility locations for the define directive object
(which is the majority of the directives)
-Much easier to hide/unhide a section in the macro directive history.
-Easier to track the effects of the directives across different submodules.
llvm-svn: 178037
b6210dff97