[libcxxabi] Fix forced_unwind3.pass.cpp compilation error

Under some circumstances there is no struct _Unwind_Exception, it's just
an alias to another struct. This would result in an error like this:

libcxxabi/test/forced_unwind3.pass.cpp:50:77: error: typedef '_Unwind_Exception' cannot be referenced with a struct specifier
  static _Unwind_Reason_Code stop(int, _Unwind_Action actions, type, struct _Unwind_Exception*, struct _Unwind_Context*,
                                                                            ^
<...>/lib/clang/15.0.0/include/unwind.h:68:38: note: declared here
typedef struct _Unwind_Control_Block _Unwind_Exception; /* Alias */
                                     ^

This seems to have been an issue since the test was first added in
D109856, except that it didn't surface with Clang 14 because the code
is filtered out by the preprocessor if `__clang_major__ < 15`.

Reviewed By: danielkiss, mstorsjo, #libc_abi, ldionne

Differential Revision: https://reviews.llvm.org/D132873