This patch implements some of rth's suggestions, and changes the signature
of the hook to take the valist and type as args and return the object.

Tested x86_64-pc-linux-gnu, ia64-pc-linux-gnu, powerpc64-pc-linux-gnu.  Now
testing (slowly) on sparc-sun-solaris9; I'll commit that hook once it
finishes.  rth, can you verify that the alpha hook still works after this
patch?

Applied to trunk.

2004-06-10  Jason Merrill  <jason@redhat.com>

	* target.h (struct gcc_target): Change gimplify_va_arg_expr 
	hook signature.
	* tree-gimple.h: Adjust.
	* config/alpha/alpha.c (alpha_gimplify_va_arg): Adjust.
	* config/i386/i386.c (ix86_gimplify_va_arg): Adjust.
	Use fold_convert.
	* config/ia64/ia64.c (ia64_gimplify_va_arg): Adjust.
	* config/rs6000/rs6000.c (rs6000_gimplify_va_arg): Adjust.
	Use COMPLEX_EXPR for complex numbers.  Use fold_convert.
	* builtins.c (std_gimplify_va_arg_expr): Adjust.  Use fold_convert.
	(gimplify_va_arg_expr): Return GS_ERROR in error case.
	Gimplify valist rather than calling stabilize_va_list.