Implement unused pattern bindings, continued (#6518)

Implementation of unused pattern bindings #2022, continued.

Whereas previous PR #6460 took care of parsing, and PR #6479 prepared
the stage by using _ in some test cases, this PR has the the actual
implementation, using a simple dataflow analysis.

---------

Co-authored-by: Burak Emir <bqe@google.com>
Co-authored-by: jonmeow <jperkins@google.com>

toolchain/check/testdata/interop/cpp/function/struct.carbon

@@ -216,7 +216,7 @@ fn F(s: Cpp.N.S) {
   //@dump-sem-ir-begin
   Cpp.foo(s);
   // Check that the parameter type was imported correctly.
-  var x: Cpp.N.S;
+  var unused x: Cpp.N.S;
   //@dump-sem-ir-end
 }
 
@@ -264,7 +264,7 @@ import Cpp library "definition_in_outer_definition.h";
 fn F(s: Cpp.O.S) {
   //@dump-sem-ir-begin
   Cpp.foo(s);
-  var x: Cpp.O;
+  var unused x: Cpp.O;
   //@dump-sem-ir-end
 }