d: Add always_inline to the internal attribute table.

d: Add always_inline to the internal attribute table.

This attribute is not directly accessible from user code, rather it is
indirectly added from the @forceinline attribute.  Even so, a handler
should be present for it to prevent false positive warnings.

Said warnings are not something that could happen currently, but will
become a problem from fixing PR90136 later.

gcc/d/ChangeLog:

	* d-attribs.cc (d_langhook_common_attribute_table): Add always_inline.
	(handle_always_inline_attribute): New function.