[SPIR-V] Segfault when emitting NonSemantic.Shader.DebugInfo.100 for uninstantiated/incomplete type template parameter

Description
Compiling the snippet below with -T cs_6_9 -spirv -fspv-debug=vulkan causes a segfault.

template<typename T>
struct A {};

template<typename T>
struct B {};

[numthreads(1, 1, 1)]
void main() {
    A<B<int> > a;
}

Steps to Reproduce
Option 1: uninstantiated template specialization
https://godbolt.org/z/c6sP977oe

Option 2: incomplete type
https://godbolt.org/z/T6zc4Yarn

Actual Behavior

Program terminated with signal: SIGSEGV
Compiler returned: 139

Environment

• DXC version 1.8.2505

Analysis
The segfault occurs when trying to lower a type that has no definition, B<int> in my example.
For a composite type like B<int>, LowerTypeVisitor::lowerType calls LowerTypeVisitor::lowerStructFields, which accesses the type's definition. B<int> is uninstantiated and so it has no definition because it is not used in a context that requires a complete type.

Now, the reason that the type B<int> is being lowered to a SPIR-V type at all is that debug type information is generated for the type of the variable a, i.e. for A<B<int>>. This is done in the parameter handling of DebugTypeVisitor::lowerDebugTypeTemplate and is, as far as I can tell, where the real logical problem is.
Ideally, you would want to generate a debug type that represents this incomplete type. Unfortunately, the extension has no way to represent that, afaik. Maybe the best solution would be to just generate a debug composite type with no members and size zero to represent B. In this case, trying to lower it to a concrete, non-debug SPIR-V type would best be avoided.

Reproduce the crash with the provided HLSL template snippets using -T cs_6_9 -spirv -fspv-debug=vulkan. Start with LowerTypeVisitor::lowerType and lowerStructFields, then inspect DebugTypeVisitor::lowerDebugTypeTemplate to trace how the incomplete B type is lowered. Done means both reproduction cases complete without a segmentation fault while preserving valid debug type handling.