[Coverage] introduce class hierarchy (funcRecordReader) to support multiple...

[Coverage] introduce class hierarchy (funcRecordReader) to support multiple versions of coverage data

[resubmit after fixing build bot failures: qualify make_unique and eliminate -Wcovered-switch-default warning.
With the planned size reduction change, the coverage format version is expected to be bumped up. This patch adds necessary support such that backward compatibility can be kept with maximal code sharing. Reading different versions of coverage data just requires instantiating the reader according to the version.

No functional change is intended.

Differiential Revision: http://reviews.llvm.org/D16133

llvm-svn: 257708