docs: Use the term "whole-program devirtualization" instead of "virtual function call optimization".

The former term is probably more familiar to users. Also add references to
the command line flags used to enable the features described in the doc.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299902 91177308-0d34-0410-b5e6-96231b3b80d8