Make the preprocessing record a PPCallbacks subclass itself,

Make the preprocessing record a PPCallbacks subclass itself,
eliminating the extra PopulatePreprocessingRecord object. This will
become useful once we start writing the preprocessing record to
precompiled headers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@98966 91177308-0d34-0410-b5e6-96231b3b80d8