[llvm] [llvm][support] Show name of overlapping cl option (PR #70108)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

makes sense in concept, but an `assert` is not going to print out the name, it'll literally print out the source code within the `assert`. should have something like
```
#ifndef NDEBUG
if (...) {
  report_fatal_error(Name + " already registered", false);
}
```
instead which will actually print out the name

also please split out the formatting changes, can push that directly without review

https://github.com/llvm/llvm-project/pull/70108