Fixed Compile error

[pikamonvvs]

Dear Respectful frasercrmck,

Thanks for your contributing a good example of making own LLVM backend. While I am building your project with gcc-8 or later, I found a small compile error. The detail that I faced is the same as following. https://github.com/microsoft/DirectXShaderCompiler/issues/1692

It was about not being able to convert unique_ptr to bool type implicitly. So it was needed to be modified to convert explicitly, which was I did.

As is bool hasMD() const { return MDMap; } To be bool hasMD() const { return static_cast(MDMap); }

(Actually, I have not yet verified if certain side effects exist when using gcc 4.9.)

Best Regards, ^^