Add a unit test for the copy constructor.

I would not normally add tests like these, but the copy constructor is not
used at all in our codebase with c++11, so having this tests might prevent
breaking the c++03 build again.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198886 91177308-0d34-0410-b5e6-96231b3b80d8

unittests/Support/ErrorOrTest.cpp

@@ -23,6 +23,9 @@ TEST(ErrorOr, SimpleValue) {
   EXPECT_TRUE(a);
   EXPECT_EQ(1, *a);
 
+  ErrorOr<int> b = a;
+  EXPECT_EQ(1, *b);
+
   a = t2();
   EXPECT_FALSE(a);
   EXPECT_EQ(errc::invalid_argument, a.getError());