[utils] fix bug in diagnostic stealing logic

When replacing expected diagnostic content, we should consider
expected diagnostics targeting the same target, not targeting the line
the expected diagnostic is on.

Author: hnrklssn

test/Utils/update-verify-tests/update-existing.swift

@@ -0,0 +1,30 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: not %target-swift-frontend-verify -typecheck %t/test.swift 2>%t/output.txt
+// RUN: %update-verify-tests < %t/output.txt
+// RUN:     %target-swift-frontend-verify -typecheck %t/test.swift
+// RUN: %diff %t/test.swift %t/test.swift.expected
+
+//--- test.swift
+func foo() {
+  let a = 2 // expected-error@+1{{asdf}}
+  b = a   // expected-error@+1{{asdf}}
+}
+
+func bar() {
+  a = 2   // expected-error@+1{{asdf}}
+}
+
+//--- test.swift.expected
+func foo() {
+  // expected-note@+1{{'a' declared here}}
+  let a = 2 // expected-error@+1{{cannot find 'b' in scope; did you mean 'a'?}}
+  b = a   
+}
+
+func bar() {
+  // expected-error@+1{{cannot find 'a' in scope}}
+  a = 2   
+}
+

utils/update_verify_tests/core.py

@@ -398,7 +398,7 @@ def remove_dead_diags(lines):
             remove_line(line, lines)
         else:
             assert line.diag.is_from_source_file
-            for other_diag in line.targeting_diags:
+            for other_diag in line.diag.target.targeting_diags:
                 if (
                     other_diag.is_from_source_file
                     or other_diag.count == 0