isl_ast_build_expr.c: ast_expr_from_pw_aff: move struct documentation to struct
isl_ast_build_expr.c: ast_expr_from_pw_aff: move struct documentation to struct

The ast_expr_from_pw_aff function will be split up and rearranged
in the following commits.  Since the data structure will then be
used by several functions, it is better to put the documentation
of the structure on the structure itself.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

nicolasvasilache/isl,Meinersbur/isl,inducer/isl-mirror,Meinersbur/isl,inducer/isl-mirror,UBERTC/isl,nicolasvasilache/isl,inducer/isl-mirror,BenzoSM/isl,tobig/isl,inducer/isl-mirror,inducer/isl-mirror,nicolasvasilache/isl,nicolasvasilache/isl,PollyLabs/isl,nicolasvasilache/isl,tobig/isl,UBERTC/isl,Meinersbur/isl,PollyLabs/isl,UBERTC/isl,BenzoSM/isl,PollyLabs/isl,PollyLabs/isl,tobig/isl,UBERTC/isl,tobig/isl,Meinersbur/isl,BenzoSM/isl,BenzoSM/isl,BenzoSM/isl,PollyLabs/isl