[Headers][ARM] Allow struct _Unwind_Exception in unwind.h

Use the same approach as used in libunwind/include/unwind_arm_ehabi.h
(D89570) and GCC's unwind-arm-common.h, so that _Unwind_Exception can be
used both with and without the struct tag.

Fixes a build failure in libcxxabi/test/forced_unwind1.pass.cpp.

Reviewed By: danielkiss

Differential Revision: https://reviews.llvm.org/D139503

(cherry picked from commit 106a992)

Author: rprichard

clang/lib/Headers/unwind.h

@@ -65,7 +65,8 @@ struct _Unwind_Context;
 #if defined(__arm__) && !(defined(__USING_SJLJ_EXCEPTIONS__) || \
                           defined(__ARM_DWARF_EH__) || defined(__SEH__))
 struct _Unwind_Control_Block;
-typedef struct _Unwind_Control_Block _Unwind_Exception; /* Alias */
+typedef struct _Unwind_Control_Block _Unwind_Control_Block;
+#define _Unwind_Exception _Unwind_Control_Block /* Alias */
 #else
 struct _Unwind_Exception;
 typedef struct _Unwind_Exception _Unwind_Exception;