[clang] [Headers] Check __SEH__, when checking if ARM EHABI is implied

[mstorsjo]

Summary

ARM EHABI isn't signalled by any specific compiler builtin define,
but is implied by the lack of defines specifying any other
exception handling mechanism, __USING_SJLJ_EXCEPTIONS__ or
__ARM_DWARF_EH__.

As Windows SEH also can be used for unwinding, check for the
__SEH__ define too, in the same way.

[mstorsjo]

@MaskRay Thanks for the other reviews! This one is the last one with the same condition added.

[MaskRay]

LGTM

clang/lib/Headers/unwind.h

 typedef uintptr_t _Unwind_Ptr;
 typedef uintptr_t _Unwind_Internal_Ptr;
 typedef uint64_t _Unwind_Exception_Class;
 
 typedef intptr_t _sleb128_t;
 typedef uintptr_t _uleb128_t;
 
 struct _Unwind_Context;
-#if defined(__arm__) && !(defined(__USING_SJLJ_EXCEPTIONS__) || defined(__ARM_DWARF_EH__))
+#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 */
 #else
 struct _Unwind_Exception;
 typedef struct _Unwind_Exception _Unwind_Exception;
 #endif
 typedef enum {
   _URC_NO_REASON = 0,
 #if defined(__arm__) && !defined(__USING_SJLJ_EXCEPTIONS__) && \
-    !defined(__ARM_DWARF_EH__)
+    !defined(__ARM_DWARF_EH__) && !defined(__SEH__)
   _URC_OK = 0, /* used by ARM EHABI */
 #endif
   _URC_FOREIGN_EXCEPTION_CAUGHT = 1,
 
   _URC_FATAL_PHASE2_ERROR = 2,
   _URC_FATAL_PHASE1_ERROR = 3,
   _URC_NORMAL_STOP = 4,
 
   _URC_END_OF_STACK = 5,
   _URC_HANDLER_FOUND = 6,
   _URC_INSTALL_CONTEXT = 7,
   _URC_CONTINUE_UNWIND = 8,
 #if defined(__arm__) && !defined(__USING_SJLJ_EXCEPTIONS__) && \
-    !defined(__ARM_DWARF_EH__)
+    !defined(__ARM_DWARF_EH__) && !defined(__SEH__)
   _URC_FAILURE = 9 /* used by ARM EHABI */
 #endif
 } _Unwind_Reason_Code;
 
 typedef enum {
   _UA_SEARCH_PHASE = 1,
   _UA_CLEANUP_PHASE = 2,
 
   _UA_HANDLER_FRAME = 4,
   _UA_FORCE_UNWIND = 8,
   _UA_END_OF_STACK = 16 /* gcc extension to C++ ABI */
 } _Unwind_Action;
 
 typedef void (*_Unwind_Exception_Cleanup_Fn)(_Unwind_Reason_Code,
                                              _Unwind_Exception *);
 
-#if defined(__arm__) && !(defined(__USING_SJLJ_EXCEPTIONS__) || defined(__ARM_DWARF_EH__))
+#if defined(__arm__) && !(defined(__USING_SJLJ_EXCEPTIONS__) || \
+                          defined(__ARM_DWARF_EH__) || defined(__SEH__))
 typedef struct _Unwind_Control_Block _Unwind_Control_Block;
 typedef uint32_t _Unwind_EHT_Header;
 
 struct _Unwind_Control_Block {
   uint64_t exception_class;
   void (*exception_cleanup)(_Unwind_Reason_Code, _Unwind_Control_Block *);
   /* unwinder cache (private fields for the unwinder's use) */
   struct {
typedef _Unwind_Reason_Code (*_Unwind_Personality_Fn)(int, _Unwind_Action,
                                                       _Unwind_Exception_Class,
                                                       _Unwind_Exception *,
                                                       struct _Unwind_Context *);
 typedef _Unwind_Personality_Fn __personality_routine;
 
 typedef _Unwind_Reason_Code (*_Unwind_Trace_Fn)(struct _Unwind_Context *,
                                                 void *);
 
-#if defined(__arm__) && !(defined(__USING_SJLJ_EXCEPTIONS__) || defined(__ARM_DWARF_EH__))
+#if defined(__arm__) && !(defined(__USING_SJLJ_EXCEPTIONS__) ||                \
+                          defined(__ARM_DWARF_EH__) || defined(__SEH__))
 typedef enum {
   _UVRSC_CORE = 0,        /* integer register */
   _UVRSC_VFP = 1,         /* vfp */
   _UVRSC_WMMXD = 3,       /* Intel WMMX data register */
   _UVRSC_WMMXC = 4,       /* Intel WMMX control register */
   _UVRSC_PSEUDO = 5       /* Special purpose pseudo register */
 } _Unwind_VRS_RegClass;