[clang] [WinEH] Check object unwinding in seh block only when -eha is used (PR #180108)

[Arthur Eubanks via cfe-commits]

aeubanks wrote:

perhaps updating clang/test/CodeGenCXX/exceptions-seh.cpp to check that the code can compile without `/EHa`, but without checking the generated code? that seems better than no test

https://github.com/llvm/llvm-project/pull/180108