[preprocessing record] Have the MacroDefinitions map point to the MacroDefinition...

[preprocessing record] Have the MacroDefinitions map point to the MacroDefinition object instead
its index in the preprocessed entities vector.

This is because the order of the entities in the vector can change in some (uncommon) cases.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175907 91177308-0d34-0410-b5e6-96231b3b80d8