[ORC] Rename MaterializationResponsibility::delegate to replace and add a new

delegate method (and unit test).

The name 'replace' better captures what the old delegate method did: it
returned materialization responsibility for a set of symbols to the VSO.

The new delegate method delegates responsibility for a set of symbols to a new
MaterializationResponsibility instance. This can be used to split responsibility
between multiple threads, or multiple materialization methods.

llvm-svn: 336603