[utils] only steal 1 diagnostic

This fixes a bug in the diagnostic stealing logic when multiple
diagnostics target the same line, which would trigger an assert.

Author: hnrklssn

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

@@ -16,6 +16,21 @@ func bar() {
   a = 2   // expected-error@+1{{asdf}}
 }
 
+func baz() {
+  // expected-error@+2{{cannot find 'a' in scope}}
+  // expected-error@+1{{cannot find 'a'}}
+  let b = a; let c = a; // expected-error{{asdf}}
+}
+
+func qux() {
+  let b = a; let c = a; // expected-error{{asdf}}
+}
+
+func foobar() {
+  var b = 1
+  b = a; b = a; // expected-error{{asdf}}
+}
+
 //--- test.swift.expected
 func foo() {
   // expected-note@+1{{'a' declared here}}
@@ -28,3 +43,22 @@ func bar() {
   a = 2   
 }
 
+func baz() {
+  // expected-error@+3{{cannot find 'a' in scope}}
+  // expected-error@+2{{cannot find 'a'}}
+  // expected-note@+1{{'b' declared here}}
+  let b = a; let c = a; 
+}
+
+func qux() {
+  // expected-note@+2{{'b' declared here}}
+  // expected-error@+1{{cannot find 'a' in scope}}
+  let b = a; let c = a; // expected-error{{cannot find 'a' in scope; did you mean 'b'?}}
+}
+
+func foobar() {
+  // expected-note@+1 2{{'b' declared here}}
+  var b = 1
+  b = a; b = a; // expected-error 2{{cannot find 'a' in scope; did you mean 'b'?}}
+}
+

utils/update_verify_tests/core.py

@@ -409,6 +409,7 @@ def remove_dead_diags(lines):
                     continue
                 line.diag.take(other_diag)
                 remove_line(other_diag.line, lines)
+                break
 
 
 def fold_expansions(lines):