Keisuke Nishimura authored Mar 16, 2021 and LAWALL Julia committed Mar 18, 2021

This patch changes the structure of 'Attribute'. In order to
support __attribute__ later, the current Attribute needs to be
refactored into a more general structure.

SmPL's ast structure(current):
__foo => Attribute( __foo)  or MetaAttribute (__foo)

SmPL's ast structure(proposed):
__foo => Attribute( AttrName (__foo) ) or Attribute( MetaAttr (__foo) )

C's ast structure(current):
foo => Attribute( foo )

C's ast structure(proposed):
foo => Attribute( AttrName(foo) )

Signed-off-by: Keisuke Nishimura <mumumu0722@gmail.com>