Fix compile error: Use explicit conversion of unique_ptr to bool

[mfwitten]

The aging LLVM code is converting an object of type unique_ptr to
an object of type bool; this is being done implicitly, thereby
causing a compile-time error with a modern, standards-compliant
compiler.

This is explained well here:

https://github.com/microsoft/DirectXShaderCompiler/issues/1692

In particular:

The C++ standard specifies operator bool on unique_ptr as being
explicit, so this is clearly a bug in the LLVM source code:
https://en.cppreference.com/w/cpp/memory/unique_ptr/operator_bool

This commit merely makes the conversion explicit, allowing the code
to be built by GCC version 9.1.0.

This bug was fixed by the LLVM project proper as part of the
following commit:

69341e6abca92f7f118ee7bd99be0cdfc649386f
2016-04-08
https://github.com/llvm/llvm-project/commit/69341e6abca92f7f118ee7bd99be0cdfc649386f

[mfwitten]

Really, the LLVM base for this project should be updated.
In the meantime, this commit keeps things working.

[bader]

@mfwitten, active development of this tool/library has moved to https://github.com/KhronosGroup/SPIRV-LLVM-Translator.
This repo is not maintained anymore, AFAIK.

[mfwitten]

Isn't it against the CoC to waste everybody's fscking time? Fix your links/website, you goofballs.

[pierremoreau]

@bader Do you know who could answer whether this is still maintained, maybe bring it up on the tooling call?

If it is no longer maintained, the README and/or description should be updated to say so (and point to SPIRV-LLVM-Translator), and the repo archived, imho.