PR c++/30277 - int-width bit-field promotion.

PR c++/30277 - int-width bit-field promotion.

Here, if cp_perform_integral_promotions saw that the TREE_TYPE of a
bit-field reference was the same as the type it promotes to, it didn't do
anything.  But then decay_conversion saw that the bit-field reference was
unchanged, and converted it to its declared type.  So I needed to add
something to make it clear that promotion has been done.  But then the 33819
change caused trouble by looking through the NOP_EXPR I just added.  This
was the wrong fix for that bug; I've now fixed that better by recognizing in
cp_perform_integral_promotions that we won't promote a bit-field larger than
32 bits, so we should use the declared type.

	PR c++/33819 - long bit-field promotion.
	* typeck.c (cp_perform_integral_promotions): Handle large bit-fields
	properly.  Handle 32-bit non-int bit-fields properly.
	(is_bitfield_expr_with_lowered_type): Don't look through NOP_EXPR.

From-SVN: r275746