Put the functionality for printing a value to a raw_ostream as an
operand into the Value interface just like the core print method is.
That gives a more conistent organization to the IR printing interfaces
-- they are all attached to the IR objects themselves. Also, update all
the users.

This removes the 'Writer.h' header which contained only a single function
declaration.

llvm-svn: 198836
GitOrigin-RevId: d48cdbf0c3ee31bcac2d8304da2ee5dc16c35056