Rework the detailed preprocessing record to separate preprocessing

entities generated directly by the preprocessor from those loaded from
the external source (e.g., the ASTReader). By separating these two
sets of entities into different vectors, we allow both to grow
independently, and eliminate the need for preallocating all of the
loaded preprocessing entities. This is similar to the way the recent
SourceManager refactoring treats FileIDs and the source location
address space.

As part of this, switch over to building a continuous range map to
track preprocessing entities.

llvm-svn: 135646