[CodeCompleiton] Don't hide members for unresolved base types
Don't filter out members if the base type has unresolved types.
Previously, initializers used to be hidden if the type has 'where'
requirements on the generic parameters.
This patch enables initializer completion for `SwiftUI.ForEach`.
rdar://problem/49480808
(cherry picked from commit 032c6e647e567f8744e9c9ef6ca595fed8541181)
