[clang] [Clang] Add support for fp when using min_fetch/max_fetch atomics (PR #160330)

[via cfe-commits]

https://github.com/CarolineConcatto commented:

I saw in this patch:

```
commit 5cf58768cb3ba31ee37facaf23f7a74f78781590
Author: Tim Northover <t.p.northover at gmail.com>
Date:   Thu Nov 21 10:31:30 2019 +0000
```

that they also added tests Sema tests:
clang/test/Sema/atomic-ops.c
Can we do the same?

I am not sure why it was not added support for floating point at the in that patch, maybe it would be nice to add as well Tim Northover as a reviewer. 

https://github.com/llvm/llvm-project/pull/160330