[LTO] Simplify internalize logic. NFC

D151965 removed incorrect internalization for {linkonce,weak}{,_odr} when
the prevailing copy is in native code. The multiple conditions are based
on negative conditions, which can be simplified to be based on positive cases:

* an object with an external linkage (must be prevailing) can be internalized
* a prevailing object with a {linkonce,weak}{,_odr} or common linkage can be internalized.

Further, the lengthy comment is a bit misleading, as it doesn't say that
objects with an external/linkonce/weak linkage can be internalized.
Clarify it.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D158949