[PassTiming] cleaning up legacy PassTimingInfo interface. NFCI.

During D51276 discussion it was decided that legacy PassTimingInfo
interface can not be reused for new pass manager's implementation
of -time-passes.

This is a cleanup in preparation for D51276 to make legacy interface
as concise as possible, moving the PassTimingInfo from the header
into the anonymous legacy namespace in .cpp.

It is rather close to a revert of rL340872 in a sense that it hides
the interface and gets rid of templates. However as compared to
a complete revert it resides in a different translation unit and has
an additional pass-instance counting funcitonality (PassIDCountMap).

Reviewers: philip.pfaffe
Differential Revision: https://reviews.llvm.org/D52356

llvm-svn: 343104