The diagnostic err_init_conversion_failed uses the enum

InitializedEntity::EntityKind as an index for one of its %select.  Over time,
EntityKind has been expanded, but the diagnostic text has not been updated.
This adds additional text to the %select to cover the new enum values.  A
comment has been added to the end of the enum regarding this situation.  This
fixes the crasher in PR17043.

llvm-svn: 189716