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

For this specific case, when catching a pointer data type, by reference,
Clang generates a special code pattern, which directly accesses the
exception data by skipping past the `_Unwind_Exception` manually (rather
than using the return value of `__cxa_begin_catch`).

On most platforms, `_Unwind_Exception` is 32 bytes, but in some
configurations it's different. (ARM EHABI is one preexisting case.) In
the case of SEH, it's also different - it is 48 bytes in 32 bit mode and
64 bytes in 64 bit mode. (See the SEH ifdef in `_Unwind_Exception` in
`clang/lib/Headers/unwind.h`.)

Handle this case in `TargetCodeGenInfo::getSizeOfUnwindException`,
fixing the code generation for catching pointers by reference.

This fixes https://github.com/mstorsjo/llvm-mingw/issues/522.

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