Fix memory leak in DeclTest.

Fixes a leak introduced in r372903, detected on the ASan bot.
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fast/builds/35430/steps/check-clang%20asan/logs/stdio

Direct leak of 192 byte(s) in 1 object(s) allocated from:
    #0 0x561d88 in operator new(unsigned long) /b/sanitizer-x86_64-linux-fast/build/llvm-project/compiler-rt/lib/asan/asan_new_delete.cc:105
    #1 0x1a48779 in clang::ItaniumMangleContext::create(clang::ASTContext&, clang::DiagnosticsEngine&) /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/lib/AST/ItaniumMangle.cpp:5134:10
    #2 0xdff000 in Decl_AsmLabelAttr_Test::TestBody() /b/sanitizer-x86_64-linux-fast/build/llvm-project/clang/unittests/AST/DeclTest.cpp:97:23

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@372925 91177308-0d34-0410-b5e6-96231b3b80d8

Author: eugenis

unittests/AST/DeclTest.cpp

@@ -94,7 +94,8 @@ TEST(Decl, AsmLabelAttr) {
 
   // Mangle the decl names.
   std::string MangleF, MangleG;
-  MangleContext *MC = ItaniumMangleContext::create(Ctx, Diags);
+  std::unique_ptr<ItaniumMangleContext> MC(
+      ItaniumMangleContext::create(Ctx, Diags));
   {
     llvm::raw_string_ostream OS_F(MangleF);
     llvm::raw_string_ostream OS_G(MangleG);