[PGO] Cleanup: Use covmap header definition in the template file

This is one last remaining instrumentatation related structure
that needs to be migrate to use the centralized template
definition.  With this change, instrumentation code 
related to coverage module header will be kept in sync
with the coverage mapping reader. The remaining code
which makes implicit assumption about covmap control
structure layout in the the lowering pass will cleaned
up in a different patch. This patch is not intended to
have no functional change.

llvm-svn: 256715