[CFI] Disable CFI checks for __cxa_decrement_exception_refcount

[vlad.tsyrklevich]

Summary

exception_header->exceptionDestructor is a void(*)(void*) function
pointer; however, it can point to destructors like std::
exception::~exception that don't match that type signature.

libcxxabi/trunk/include/__cxxabi_config.h

 #endif
 
 #if defined(_WIN32)
 #define _LIBCXXABI_WEAK
 #else
 #define _LIBCXXABI_WEAK __attribute__((__weak__))
 #endif
 
+#if defined(__clang__)
+#define _LIBCXXABI_COMPILER_CLANG
+#endif
+
+#if __has_attribute(__no_sanitize__) && defined(_LIBCXXABI_COMPILER_CLANG)
+#define _LIBCXXABI_NO_CFI __attribute__((__no_sanitize__("cfi")))
+#else
+#define _LIBCXXABI_NO_CFI
+#endif
+
 #endif // ____CXXABI_CONFIG_H

libcxxabi/trunk/src/cxa_exception.cpp

 /*
     If thrown_object is not null, atomically decrement the referenceCount field
     of the __cxa_exception header associated with the thrown object referred to
     by thrown_object.  If the referenceCount drops to zero, destroy and
     deallocate the exception.
 
     Requires:  If thrown_object is not NULL, it is a native exception.
 */
-void
-__cxa_decrement_exception_refcount(void *thrown_object) throw() {
+_LIBCXXABI_NO_CFI
+void __cxa_decrement_exception_refcount(void *thrown_object) throw() {
     if (thrown_object != NULL )
     {
         __cxa_exception* exception_header = cxa_exception_from_thrown_object(thrown_object);
         if (__sync_sub_and_fetch(&exception_header->referenceCount, size_t(1)) == 0)
         {
             if (NULL != exception_header->exceptionDestructor)
                 exception_header->exceptionDestructor(thrown_object);
             __cxa_free_exception(thrown_object);