spirv: fix build for C++20 (#6105)

This codebase is using C++17, but to remain compatible with C++20, I
need to remove this.
Starting C++20, an aggregate is required to have no user-declared
constructors, vs before, no user-provided constructors.

So even if if the only the default constructor is provided, this is
enough to prevent us from using this class as an aggregate in c++20.

Signed-off-by: Nathan Gauër <brioche@google.com>

tools/clang/lib/SPIRV/DeclResultIdMapper.h

@@ -384,8 +384,6 @@ class DeclResultIdMapper {
   /// The struct containing the data needed to create the input and output
   /// variables for the decl.
   struct StageVarDataBundle {
-  public:
-    StageVarDataBundle() = default;
     // The declaration of the variable for which we need to create the stage
     // variables.
     const NamedDecl *decl;