Add const qualification to symbol accesses

All accesses to built-in symbols now happen through const-qualified
pointers.

This also encapsulates TSymbolTableLevel inside TSymbolTable.

This prepares for statically allocating built-in symbols.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I473014d978daa765b4a733d761d6c08b28288776
Reviewed-on: https://chromium-review.googlesource.com/859959
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/compiler/translator/OutputGLSLBase.cpp b/src/compiler/translator/OutputGLSLBase.cpp
index ec4a65a..b3e8476 100644
--- a/src/compiler/translator/OutputGLSLBase.cpp
+++ b/src/compiler/translator/OutputGLSLBase.cpp
@@ -260,7 +260,7 @@
     }
     if (type.getBasicType() == EbtInterfaceBlock)
     {
-        TInterfaceBlock *interfaceBlock = type.getInterfaceBlock();
+        const TInterfaceBlock *interfaceBlock = type.getInterfaceBlock();
         declareInterfaceBlockLayout(interfaceBlock);
     }
     if (qualifier != EvqTemporary && qualifier != EvqGlobal)
@@ -317,7 +317,7 @@
     }
     else if (type.getBasicType() == EbtInterfaceBlock)
     {
-        TInterfaceBlock *interfaceBlock = type.getInterfaceBlock();
+        const TInterfaceBlock *interfaceBlock = type.getInterfaceBlock();
         declareInterfaceBlock(interfaceBlock);
     }
     else