[GLSL] `--flatten-ubo` with preserving names

Hi! Is it possible to flatten UBO with preserving member names? Or it's too complicated to support?

**GLSL (source) > SPIRV**:
```glsl
#version 450

layout (location = 0) uniform Constants
{
vec4 color;
// ...
};

layout (location = 0) out vec4 outColor;

void main()
{
outColor = color;
}
```

**SPIRV > GLSL (result)**:
```glsl
#version 110

// Instead of uniform vec4 Constants[...];
uniform vec4 color;
// uniform ...

void main()
{
gl_FragColor = color; // instead of Constants[0];
}
```

Start by tracing how the --flatten-ubo option handles UBO members during SPIR-V to GLSL translation. Compare the current flattened output with the requested preservation of member names in the issue example, then determine the required behavior and validation coverage. Done means flattened UBOs retain usable member names without breaking generated GLSL.

Written by the indexing model from the issue text.