[clang] [clang] Fix catching pointers by reference on mingw targets (PR #162546)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

> (See the SEH ifdef in `_Unwind_Exception` in clang/lib/Headers/unwind.h.)

The header also checks that `__USING_SJLJ_EXCEPTIONS__` is not defined.  Is that just redundant?

https://github.com/llvm/llvm-project/pull/162546