[clang] [RISCV] Enable target attribute when invoked through clang driver (PR #74889)

[Philip Reames via cfe-commits]

preames wrote:

> Related question. If there is an -mcpu on the command line and target attribute changes the march, do we keep the original CPU in the -target-cpu attribute or drop it. The reason for all those negative features from the driver was to make the backend not infer any features from the CPU that weren't in the provided march. So I'm wondering if we have that issue with the target attribute now.

It looks like the original -mcpu from the command line is passed through the to the function attributes unless the *cpu* is overridden via the target attribute.  Doing a full replacement of the march does not appear to change or remove the target-cpu attribute on the result.  

One case I could see being problematic would be a CPU which enabled e.g. V, and a replacement march which didn't.  

Before this change, we'd end up with with an explicit target-feature for +v (coming from the original list passed into cc1), and that's clearly wrong.  After this change, we end up with the explicit features being entirely missing (i.e. no explicit +v).

I had to go run a test to see what happened from there.  It does look like we re-infer from the cpu definition and proceed to generate vector code.  Yeah, that's more than a bit suspect.

This does fall into the "extension subtraction" case we'd tried to leave out of the specification.  So I can see two fixes here: add back the explicit negative feature, or update the specification text to disallow this.  

https://github.com/llvm/llvm-project/pull/74889