Fix build break when VS2015 is not installed. (#14948)

Fix build break when VS2015 is not installed. (#14948)

* Fix build break when VS2015 is not installed.

If you ONLY install VS2017 (now the common scneario)
and build coreclr, it will fail  with COM with a Class_Not_Registered error

This is because a build-tools package is depending on an old COM object
that has gone way in VS2017.

While we sort getting a proper fix, this makes the work-around (which
is registering the DLL by hand) discoverable.

See https://github.com/dotnet/coreclr/issues/11305 for details.

* typo