[clang] [clang] Add support for `__declspec(no_init_all)` (PR #116847)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

I see.  I agree we don't need to add non-__declspec syntax here.  We're adding this for MSVC compat, so we need to match MSVC semantics anyway, so there's not really any other way to implement this.  We can always adjust the code later if we end up implementing an overlapping feature for non-MSVC targets.

I'd still like to have a basic description in documentation for reference, especially since there isn't any corresponding MSVC compiler documentation.

https://github.com/llvm/llvm-project/pull/116847