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

[Fangrui Song via llvm-commits]

MaskRay wrote:

Best to add `.section` parser 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.

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