Using clang-cl 64 bit on windows 8.1

I'v just started using clang-cl on windows, because I need to use inline assembly in 64 bit app, and Visual Studio 2015 doesn't support that, so I was told to go for clang-cl.

I downloaded a pre-built binary (clang 3.7.0) from here, the windows 64 bit version.

So I tried to make my first program, but sadly it doesn't compile. The same code does compile on Visual Studio 2015 (except the assembly statement).

Please help

This is the command I used from admin cmd (the I ran on it vsvart32 just before that):
clang-cl.exe -m64 C:\test\Source.cpp

Here is my code:

#include <Windows.h>
#include <iostream>

int main() {

    int a = 0;
    if(0)//this if-else is to mess up disassmblers
        __asm __emit 0xE8 //only this line doesn't go on VS2015
    else
    a=3;

    if (IsDebuggerPresent())
    MessageBox(
        NULL,
        (LPCWSTR)L"Debugger detected!!",
        (LPCWSTR)L"!!!!!",
        MB_ICONWARNING | MB_CANCELTRYCONTINUE | MB_DEFBUTTON2
        );
    return 0;
}

And this is the clang-cl.exe output:

C:\Program Files\LLVM\bin>clang-cl.exe -m64 C:\test\Source.cpp
C:\test\Source.cpp(13,2) :  error: no matching function for call to
  'MessageBoxA'
    MessageBox(
    ^~~~~~~~~~
C:\Program Files (x86)\Windows Kits\8.1\include\um\winuser.h(8705,21) :       note:
  expanded from macro 'MessageBox'
#define MessageBox  MessageBoxA
                ^~~~~~~~~~~
C:\Program Files (x86)\Windows Kits\8.1\include\um\winuser.h(8689,1) :  note:
  candidate function not viable: no known conversion from 'LPCWSTR'
  (aka 'const wchar_t *') to 'LPCSTR' (aka 'const char *') for 2nd argument
MessageBoxA(
^
1 error generated.

EDIT thanks to @Martin Bonner , the problem was I needed to use #define UNICODE. But now I also need to compile on 64bit. How do I do that?

You need #define UNICODE before #include <windows.h> if you are going to pass wide strings to functions like MessageBox