Follow-up for D74006.
When the integrated assembler is used, we use SHF_LINK_ORDER. The
linked-to symbol is part of ELFSectionKey, thus we can omit the unique
ID.
https://bugs.llvm.org/show_bug.cgi?id=44775
This rule has been implemented by GNU as https://sourceware.org/ml/binutils/2020-02/msg00028.html (binutils >= 2.35)
It allows us to simplify
```
.section .foo,"o",foo,unique,0
.section .foo,"o",bar,unique,1 # different section
```
to
```
.section .foo,"o",foo
.section .foo,"o",bar # different section
```
We consider the two `.foo` different even if the linked-to symbols foo and bar
are defined in the same section. This is a deliberate choice so that we don't
need to know the section where foo and bar are defined beforehand.
Differential Revision: https://reviews.llvm.org/D74006
