libsepol/cil: Check for statements not allowed in optional blocks

While there are some checks for invalid statements in an optional
block when resolving the AST, there are no checks when building the
AST.

OSS-Fuzz found the following policy which caused a null dereference
in cil_tree_get_next_path().
  (blockinherit b3)
  (sid SID)
  (sidorder(SID))
  (optional o
    (ibpkeycon :(1 0)s)
    (block b3
      (filecon""block())
      (filecon""block())))

The problem is that the blockinherit copies block b3 before
the optional block is disabled. When the optional is disabled,
block b3 is deleted along with everything else in the optional.
Later, when filecon statements with the same path are found an
error message is produced and in trying to find out where the block
was copied from, the reference to the deleted block is used. The
error handling code assumes (rightly) that if something was copied
from a block then that block should still exist.

It is clear that in-statements, blocks, and macros cannot be in an
optional, because that allows nodes to be copied from the optional
block to somewhere outside even though the optional could be disabled
later. When optionals are disabled the AST is reset and the
resolution is restarted at the point of resolving macro calls, so
anything resolved before macro calls will never be re-resolved.
This includes tunableifs, in-statements, blockinherits,
blockabstracts, and macro definitions. Tunable declarations also
cannot be in an optional block because they are needed to resolve
tunableifs. It should be fine to allow blockinherit statements in
an optional, because that is copying nodes from outside the optional
to the optional and if the optional is later disabled, everything
will be deleted anyway.

Check and quit with an error if a tunable declaration, in-statement,
block, blockabstract, or macro definition is found within an
optional when either building or resolving the AST.

Signed-off-by: James Carter <jwcart2@gmail.com>