Fix GLSL global const diagnostic regression

[Copilot]

This PR fixes a regression introduced by PR #7711 where the diagnostic for global const variables with initializers was incorrectly triggering in GLSL mode.

Problem

In GLSL mode, global non-static const variables represent real constants, not uniform parameters. However, the diagnostic added in PR #7711 was treating them the same as HLSL, causing false positive errors when compiling valid GLSL code:

// This should be valid in GLSL mode but was incorrectly flagging error 31224
const float PI = 3.14159;
const vec3 GRAVITY = vec3(0.0, -9.8, 0.0);

Solution

Added a condition to exclude GLSL modules from the diagnostic check in slang-check-decl.cpp:

// Before: Always diagnosed global const with initializers
if (isGlobalDecl(decl) && (hasConst || hasUniform) && !hasStatic &&
    !hasSpecializationConstant && decl->initExpr)

// After: Skip diagnostic for GLSL modules  
auto moduleDecl = getModuleDecl(decl);
if (!moduleDecl || !moduleDecl->hasModifier<GLSLModuleModifier>())

Testing

Added comprehensive test coverage:

• tests/diagnostics/glsl-global-const-with-init.slang - Verifies GLSL mode allows global const with initializers
• tests/diagnostics/hlsl-global-const-with-init-still-errors.slang - Ensures HLSL mode still produces the diagnostic

All existing diagnostic tests continue to pass (223/223), confirming no regressions.

Fixes #7807.

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[jkwak-work]

Wow. This looks good to me.