[clang] [Clang](NFC) Add coverage for VTable debug info (PR #151818)

[Tomohiro Kashiwada via cfe-commits]

kikairoya wrote:

> As you say, there are different outputs between -O0 and -O1, but is that because of a behaviour related to this feature, or is it just the normal behaviour of the compiler? If it's normal, we should remove the -O1 coverage. If there's something important (the presence / absence of the external symbol?) then we should narrow down what pass causes that behaviour and test that, rather than a full invocation of clang.

I've noticed a difference in behavior based on the optimization level, which changes the linkage of the vtables.

https://github.com/llvm/llvm-project/blob/c0562ff27f4de086e4687c5ad5aaea6102faf0b8/clang/lib/CodeGen/CGVTables.cpp#L1120-L1121

We definitely need both of `-O0` and `-O1`, but the latter can be `-O1 -disable-llvm-passes`. Does that make sense?

https://github.com/llvm/llvm-project/pull/151818