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/function/generic/undefined.carbon

@@ -14,7 +14,7 @@
 
 library "[[@TEST_NAME]]";
 
-fn Defined[T:! type](x: T) {}
+fn Defined[T:! type](unused x: T) {}
 
 fn CallDefined() {
   Defined(0 as i32);
@@ -30,7 +30,7 @@ fn CallDefined() {
   Defined(0 as i32);
 }
 
-fn Defined[T:! type](x: T) {}
+fn Defined[T:! type](unused x: T) {}
 
 // --- fail_call_undefined.carbon