python: Add reverse flow in some patterns

Particularly in value and literal patterns.
This is getting a little bit into the guards aspect of matching.
We could similarly add reverse flow in terms of
sub-patterns storing to a sequence pattern,
a flow step from alternatives to an-or-pattern, etc..
It does not seem too likely that sources are embedded in patterns
to begin with, but for secrets perhaps?

It is illustrated by the literal test. The value test still fails.
I believe we miss flow in general from the static attribute.

python/ql/test/experimental/dataflow/match/test.py

@@ -46,6 +46,10 @@ def test_or_pattern():
             SINK(x) #$ flow="SOURCE, l:-3 -> x"
 
 # No flow for literal pattern
+def test_literal_pattern():
+    match SOURCE:
+        case 42 as x:
+            SINK(x) #$ flow="SOURCE, l:-2 -> x" flow="42, l:-1 -> x"
 
 def test_capture_pattern():
     match SOURCE: