-Serialize the macro directives history into its own section
-Get rid of the macro updates section
-When de/serializing an identifier from a module, associate only one macro per
 submodule that defined+exported it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177761 91177308-0d34-0410-b5e6-96231b3b80d8