[CSClosure] Diagnose invalid re-declarations in multi-statement closures

Calling `typeCheckDecl` on `VarDecl` is what triggers re-declaration
checking and that was skipped by the solution application logic.

(cherry picked from commit 854f64e)

Author: xedin

lib/Sema/CSClosure.cpp

@@ -1177,10 +1177,6 @@ class ClosureConstraintApplication
     if (isa<IfConfigDecl>(decl))
       return;
 
-    // Variable declaration would be handled by a pattern binding.
-    if (isa<VarDecl>(decl))
-      return;
-
     // Generate constraints for pattern binding declarations.
     if (auto patternBinding = dyn_cast<PatternBindingDecl>(decl)) {
       SolutionApplicationTarget target(patternBinding);

test/expr/closure/multi_statement.swift

@@ -234,6 +234,20 @@ func test_local_function_capturing_vars() {
   }
 }
 
+func test_test_invalid_redeclaration() {
+  func test(_: () -> Void) {
+  }
+
+  test {
+    let foo = 0 // expected-note {{'foo' previously declared here}}
+    let foo = foo // expected-error {{invalid redeclaration of 'foo'}}
+  }
+
+  test {
+    let (foo, foo) = (5, 6) // expected-error {{invalid redeclaration of 'foo'}} expected-note {{'foo' previously declared here}}
+  }
+}
+
 func test_pattern_ambiguity_doesnot_crash_compiler() {
   enum E {
   case hello(result: Int) // expected-note 2 {{found this candidate}}