Quick fix for off-by-count errors in pattern binding diagnostics

Rather than return the last var pattern, only match on the ones that
match the vardecl the diagnostic is warning about.

lib/Sema/MiscDiagnostics.cpp

@@ -2315,7 +2315,8 @@ VarDeclUsageChecker::~VarDeclUsageChecker() {
         VarPattern *foundVP = nullptr;
         pattern->forEachNode([&](Pattern *P) {
           if (auto *VP = dyn_cast<VarPattern>(P))
-            foundVP = VP;
+            if (VP->getSingleVar() == var)
+              foundVP = VP;
         });
 
         if (foundVP && !foundVP->isLet())

test/decl/var/usage.swift

@@ -48,6 +48,21 @@ class TestClass {
   }
 }
 
+enum TestEnum {
+  case Test(Int, Int, Int)
+}
+
+func testEnum() -> Int {
+  let ev = TestEnum.Test(5, 6, 7)
+  switch ev {
+  case .Test(var i, var j, var k): // expected-warning {{variable 'i' was never mutated; consider changing to 'let' constant}} {{14-17=let}}
+                                   // expected-warning@-1 {{variable 'j' was never mutated; consider changing to 'let' constant}} {{21-24=let}}
+                                   // expected-warning@-2 {{variable 'k' was never mutated; consider changing to 'let' constant}} {{28-31=let}}
+    return i + j + k
+  default:
+    return 0
+  }
+}
 
 func nestedFunction() -> Int {
   var x = 42  // No warning about being never-set.
@@ -231,4 +246,3 @@ func test(_ a : Int?, b : Any) {
 
 }
 
-