[clang] [llvm] IR: Store the default subtarget info in named metadata instead of the context. (PR #98673)

[via llvm-commits]

pcc wrote:

This is a WIP. Because I'm adding new metadata, several tests fail, e.g. check-clang:
```
Testing Time: 244.38s

Total Discovered Tests: 37911
  Skipped          :    31 (0.08%)
  Unsupported      :   102 (0.27%)
  Passed           : 37689 (99.41%)
  Expectedly Failed:    28 (0.07%)
  Failed           :    61 (0.16%)
```
I'm uploading this to get agreement on the direction before updating the tests.

https://github.com/llvm/llvm-project/pull/98673