FR: Avoid 'incomplete alias created' warning if aliasing is known to be safe

[lauraharker]

See the most recent comments on #2595.

Right now CollapseProperties warns for aliasing a namespace if any properties on that namespace may be unsafely collapsed. (i.e. if there are constructors or enums on the namespace)

In cases like #2595, though, the only usage of the aliased namespace is to access a function property (which is not collapsible). The compiler should be able to determine that this is safe and not warn.

[tjgq]

Created Google internal issue b/137362836