Inliner: implement FullPolicy

Inliner: implement FullPolicy

The FullPolicy will be used to grow maximal potential inline trees
for methods. It inlines all legal candidates subject to depth and
size limits. Add a config flag to select this as the policy to use.

Also add a config flag so the inline depth limit becomes adjustable,
and rework the code so that checking this limit is now a matter of
policy.  Add in an implementation max depth limit of 1000.

Revise names of some statics to make their intent clearer, and add some
missing header comments.

Commit migrated from https://github.com/dotnet/coreclr/commit/8bed64ec2b36608367fc8c075b796b1bd34ab89f