[clang] [llvm] [MTE] Add an attribute to opt-in memory tagging of global variables while using fsanitize=memtag-globals (#166380) (PR #168535)

[Peter Collingbourne via llvm-commits]

pcc wrote:

It's confusing that we have both `SanitizerMetadata::Memtag` to enable MTE globals as well as a separate `MDNode` to force enable it. Can we make it so that the frontend sets `SanitizerMetadata::Memtag` if there is either no section attribute or your new attribute, and remove the section check from the backend?

https://github.com/llvm/llvm-project/pull/168535