[llvm] [AMDGPU][MC] Allow UC_VERSION_* constant reuse (PR #96461)

[Carl Ritson via llvm-commits]

perlfu wrote:

> LGTM, thanks! An error message or just removing the `assert(!Sym->isVariable() || ...` bit would be great.

I have made this output a warning to the MCContext.
This seems like the right thing to do as the behaviour of these messages is configurable through standard mechanisms (can be elevated to error, or suppressed entirely).

I have put back the redefinition unit test as proof that there is no crash.
It also checks that the warning message is output as expected.

https://github.com/llvm/llvm-project/pull/96461