-gVS drops DebugSource for included files when the include contributes only macros or otherwise unused code

### Description
When compiling with `-gVS`, include files can be omitted from `NonSemantic.Shader.DebugInfo.100` source records if they are not referenced by any DebugLine or variable/type declaration. This happens with headers that contain only macros or helper functions that are never used by the main shader. This is not an issue when compiling with `-g`, where every include file has an `OpSource` regardless of usage.

### Motivation
I am trying to use non semantic debug info to re-compile shaders. This fails when the source for some include files is missing.

### Expected Behavior
I would expect a `DebugSource` record to be emitted for each non-empty included file known to the compiler.

### Simple Repro
test.frag:
```
#version 450
#extension GL_GOOGLE_include_directive : enable

#include "utils.glsl"

layout(location = 0) out vec4 fragColor;

void main() {
fragColor = vec4(COLOR_R, COLOR_G, 0, 0);
}
```
utils.glsl:
```
#include "constants.h"

float unused_luminance(in vec3 color)
{
return dot(color, vec3(LUMINANCE_R, LUMINANCE_G, LUMINANCE_B));
}
```
constants.h:
```
#define COLOR_R 0.0
#define COLOR_G 1.0

#define LUMINANCE_R 0.299
#define LUMINANCE_G 0.587
#define LUMINANCE_B 0.114
```
Compile with:
`glslang -V -gVS -o test.spv test.frag`

Observe that only `test.frag` has `DebugSource`. Change `-gVS` to `-g` and observe that every file now has `OpSource`.

### Possible Approach
Looking at `SpvBuilder.cpp`, I can see that `Builder::dumpSourceInstructions` loops through all include files and emits source records for the `-g` case. However, in the `-gVS` case, `Builder::makeDebugCompilationUnit` and `Builder::makeDebugSource` lazily emit debug source records only when a debug record refers to that file. Since `makeDebugSource` already guards against duplicates, it seems like a loop over all the include in the non semantic case could work.

Start in SpvBuilder.cpp, comparing dumpSourceInstructions with the lazy makeDebugCompilationUnit and makeDebugSource paths used for -gVS. Run the provided test.frag, utils.glsl, and constants.h reproduction with -gVS and -g, then inspect the generated source records. Done means every non-empty included file receives a DebugSource record under -gVS, including unused macro-only or helper-code files.

Written by the indexing model from the issue text.