Add a comment describing the purpose of the NativeHalfType conditional

Author: Icohedron

clang/lib/CodeGen/CGHLSLRuntime.cpp

@@ -287,6 +287,10 @@ void CGHLSLRuntime::finishCodeGen() {
   Triple T(M.getTargetTriple());
   if (T.getArch() == Triple::ArchType::dxil)
     addDxilValVersion(TargetOpts.DxilValidatorVersion, M);
+
+  // NativeHalfType corresponds to the -fnative-half-type clang option which is
+  // aliased by clang-dxc's -enable-16bit-types option. This option is used to
+  // set the UseNativeLowPrecision DXIL module flag in the DirectX backend
   if (LangOpts.NativeHalfType)
     M.setModuleFlag(llvm::Module::ModFlagBehavior::Error, "dx.nativelowprec",
                     1);