Add "override" and remove "virtual" where appropriate.

For the record, I used clang-modernize to add "override" and perl to remove
"virtual".

llvm-svn: 203164
diff --git a/lld/unittests/DriverTests/DarwinLdDriverTest.cpp b/lld/unittests/DriverTests/DarwinLdDriverTest.cpp
index 6c8799e..33d9367 100644
--- a/lld/unittests/DriverTests/DarwinLdDriverTest.cpp
+++ b/lld/unittests/DriverTests/DarwinLdDriverTest.cpp
@@ -26,7 +26,7 @@
 class DarwinLdParserTest
     : public ParserTest<DarwinLdDriver, MachOLinkingContext> {
 protected:
-  virtual const LinkingContext *linkingContext() { return &_context; }
+  const LinkingContext *linkingContext() override { return &_context; }
 };
 
 TEST_F(DarwinLdParserTest, Basic) {