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In GCC 8, gcc -fsanitize=undefined flags the undefined behavior
that Emacs relies on in its XPNTR and XSYMBOL low-level functions.
Disable undefined sanitization in these functions.  Although this
disabling doesn’t suffice if DEFINE_KEY_OPS_AS_MACROS is true, it
works for -fsanitize=undefined -DINLINING=0, which is good enough.
* src/alloc.c (macro_PNTR_ADD): New macro.
(PNTR_ADD): New function and macro.
The function disables -fsanitize=undefined.
(macro_XPNTR): Use it.
* src/conf_post.h (ATTRIBUTE_NO_SANITIZE_UNDEFINED): New macro.
* src/lisp.h (XSYMBOL): Disable -fsanitize=undefined.