Unify DependencyFileGenerator class and DependencyCollector interface (NFCI)

Make DependencyFileGenerator a DependencyCollector as it was intended when
DependencyCollector was introduced. The missing PPCallbacks overrides are added to
the DependencyCollector as well.

This change will allow clang-scan-deps to access the produced dependencies without
writing them out to .d files to disk, so that it will be able collate them and
report them to the user.

Differential Revision: https://reviews.llvm.org/D63290

llvm-svn: 363840