[Coverage] Refactor coverage mapping reader code /NFC

(Resubmit after fixing build bot failures)

In this refactoring, member functions are introduced to access
CovMap header/func record members and hide layout details. This
will enable further code restructuring to support reading multiple
versions of coverage mapping data with shared/templatized code. 
(When coveremap format version changes, backward compatibtility
should be preserved).

llvm-svn: 257551