Remove unneeded template alias, causes issues with some MSVC version

I built locally with the latest MSVC in c++14 and c++17, but it does not
complain for me. Osman Zakir on llvm-dev reports that they run into
compile errors here.

In any case, it seems prefereable to reuse clang's LLVM.h header to
bring in llvm::Optional and Expected.

clang/lib/AST/Interp/ByteCodeStmtGen.cpp

@@ -14,13 +14,11 @@
 #include "PrimType.h"
 #include "Program.h"
 #include "State.h"
+#include "clang/Basic/LLVM.h"
 
 using namespace clang;
 using namespace clang::interp;
 
-template <typename T> using Expected = llvm::Expected<T>;
-template <typename T> using Optional = llvm::Optional<T>;
-
 namespace clang {
 namespace interp {