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[cc1] Add warnings about promotable types in va_arg()

Bool, char  and short are promoted to int when they are passed through ...,
and it means that it is impossible to have them in a stack created by a
variadic function. This patch warns to the user about it and correct
the type to int.

--- a/cc1/builtin.c

+++ b/cc1/builtin.c

@@ -20,6 +20,12 @@

 		errorp("incorrect parameters for va_arg");

 		return constnode(zero);

 	}

+	if (tp == booltype ||

+	    tp == chartype || tp == uchartype || tp == schartype ||

+	    tp == shortype || tp == ushortype) {

+		warn("bool, char and short are promoted to int when passed through '...'");

+		tp = (tp->prop & TSIGNED) ? inttype : uinttype;

+	}

 	np = node(OBUILTIN, tp, ap, NULL);

 	np->sym = sym;