[llvm] [COFF] Preserve UniqueID used to create MCSectionCOFF (PR #123869)

[Haohai Wen via llvm-commits]

HaohaiWen wrote:

> Best to add `.section` parser and printer support so that this feature is testable. The new test can be added to llvm/test/MC/COFF/section-unique.s
> 
> @mstorsjo might be interested in ensuring gas compatibility.

Is there any specification about COFF .section directive?
Current format is 
```.section section_name,"characteristic"[(,selection,comdat_name) | (\n.linkonce selection)]```
If we can define .section directive format, I'd like to define it as
```.section section_name,"characteristic[,unique,unique_id]"[(,selection,comdat_name) | (\n.linkonce selection)]```
Any suggestion about that?

https://github.com/llvm/llvm-project/pull/123869