[clang] [clang] add unnamed_addr function attribute (PR #92499)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

I think the underlying functionality is pretty clearly useful: identical code folding gives significant codesize reductions. In fact, on Windows, the linker does this kind of folding automatically by default (despite the fact that it isn't standards-compliant).  I imagine if we had this implemented, libc++ would want to use it.

https://github.com/llvm/llvm-project/pull/92499