PY-21175 Add write instructions for assignments to attributes in CFG

to handle, in particular, lazy initialization scenarios

For instance, the statement "foo.bar = 42" should add a node
"WRITE ACCESS foo.bar" containing the corresponding target expression.

Additionally, "Unused local" inspection doesn't consider writes to
attributes, since, strictly speaking, these are not local names.

python/testData/codeInsight/controlflow/qualifiedselfreference.txt

@@ -2,5 +2,6 @@
 1(2) WRITE ACCESS: self
 2(3) WRITE ACCESS: args
 3(4) element: PyAssignmentStatement
-4(5) READ ACCESS: self
-5() element: null
+4(5) WRITE ACCESS: self.args
+5(6) READ ACCESS: self
+6() element: null

python/testData/codeInsight/controlflow/self.txt

@@ -1,7 +1,8 @@
 0(1) element: null
 1(2) WRITE ACCESS: self
 2(3) element: PyAssignmentStatement
-3(4) READ ACCESS: self
-4(5) element: PyPrintStatement
-5(6) READ ACCESS: self
-6() element: null
+3(4) WRITE ACCESS: self.foo
+4(5) READ ACCESS: self
+5(6) element: PyPrintStatement
+6(7) READ ACCESS: self
+7() element: null