c-family: attribute ((aligned, mode)) [PR100545]

The problem here was that handle_mode_attribute clobbered the changes of any
previous attribute, only copying type qualifiers to the new type.  And
common_handle_aligned_attribute had previously set up the typedef, so when
we later called set_underlying_type it saw DECL_ORIGINAL_TYPE set and just
returned, even though handle_mode_attribute had messed up the TREE_TYPE.
So, let's fix handle_mode_attribute to copy attributes, alignment, and
typedefness to the new type.

	PR c/100545

gcc/c-family/ChangeLog:

	* c-attribs.cc (handle_mode_attribute): Copy attributes, aligned,
	and typedef.
	* c-common.cc (set_underlying_type): Add assert.

gcc/testsuite/ChangeLog:

	* c-c++-common/attr-mode-1.c: New test.
	* c-c++-common/attr-mode-2.c: New test.