Do not suppress 'uses' node effects

Do not suppress 'uses' node effects

Parser tries to process an 'augment' statement with 'if-feature' and
fails when the feature is not supported and the 'augment' statement
targets a node defined with a nested grouping and dependent on the
feature.

Add a test case (4th one) to reproduce the issue. The other test cases
show that there is no parsing issue if the augmentation target node is
defined with a single grouping or feature is supported.

The problem here is not AugmentStatementSupport, but rather
UsesStatementSupport, which fails to propagate children when if-feature
is in effect -- hence those children never make it to the schema tree,
and hence fail to be looked up.

Use 'uses' statement's if-feature to mark any children as unsupporte,
hence they are reflected in namespaces, but they will not be built.

JIRA: YANGTOOLS-1471
Change-Id: Ibd86530388a99ed3c8ade7bf9af109c6a0cfb80a
Signed-off-by: Sangwook Ha <sangwook.ha@verizon.com>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>