Correct modifier isConst initialization (shader-slang#6645)

The previous implementation had two issues in the modifier processing loop:
1. isConst was incorrectly initialized to true, making the const check redundant
2. Premature loop break could skip processing important modifiers. e.g.
   isExtern

Changes:
- Initialize isConst to false by default
- Remove early break condition to process all modifiers

Fixes: shader-slang#6606

Co-authored-by: Yong He <yonghe@outlook.com>

Author: gtong-nv

source/slang/slang.cpp

@@ -2879,7 +2879,7 @@ static void collectExportedConstantInContainer(
         if (!varMember->val)
             continue;
         bool isExported = false;
-        bool isConst = true;
+        bool isConst = false;
         bool isExtern = false;
         for (auto modifier : m->modifiers)
         {
@@ -2892,8 +2892,6 @@ static void collectExportedConstantInContainer(
             }
             if (as<ConstModifier>(modifier))
                 isConst = true;
-            if (isExported && isConst)
-                break;
         }
         if (isExported && isConst)
         {