[clang] [MS] Put dllexported inline global initializers in a comdat (PR #107154)

[Reid Kleckner via cfe-commits]

rnk wrote:

Yeah, I should've explicitly said I was working on updating the comment block. It needed significant surgery. Actually, this code could probably use more significant refactoring, but let's set that aside for now.

> Does this impact non-MS targets?

At the moment, I can't think of a good way to make an inline global `weak_odr` on non-MS targets without using templates, which would hit the existing `isTemplateInstantiation` check. I want to say "no" with ~85% confidence.

https://github.com/llvm/llvm-project/pull/107154