Fix TimeValue::now() on Unix.

TimeValue()::now().toEpochTime() is supposed to be the same as time(),
but it wasn't, because toEpoch subtracted PosixZeroTime, but now()
didn't add PosixZeroTime!
Add a unittest to check this works.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@94178 91177308-0d34-0410-b5e6-96231b3b80d8

unittests/Support/System.cpp

@@ -0,0 +1,16 @@
+//===- llvm/unittest/Support/System.cpp - System tests --===//
+#include "gtest/gtest.h"
+#include "llvm/System/TimeValue.h"
+#include <time.h>
+
+using namespace llvm;
+namespace {
+class SystemTest : public ::testing::Test {
+};
+
+TEST_F(SystemTest, TimeValue) {
+  sys::TimeValue now = sys::TimeValue::now();
+  time_t now_t = time(NULL);
+  EXPECT_TRUE(abs(now_t - now.toEpochTime()) < 2);
+}
+}