dawn: Fix unreachable code warning

Change-Id: I98fd18b4747365c6474dc1ca1f9003464d171ec7
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/100580
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/dawn/native/opengl/ShaderModuleGL.cpp

@@ -52,6 +52,7 @@ tint::writer::glsl::Version::Standard ToTintGLStandard(opengl::OpenGLVersion::St
         case opengl::OpenGLVersion::Standard::ES:
             return tint::writer::glsl::Version::Standard::kES;
     }
+    UNREACHABLE();
 }
 
 using BindingMap = std::unordered_map<tint::sem::BindingPoint, tint::sem::BindingPoint>;