Rename ConstantUnion to TConstantUnion.

This clarified that we're using the Pool allocator/deallocator for
this type.

BUG=angleproject:993

Change-Id: If8c95f6054d07291e7014be0d4e35766ba2e943b
Reviewed-on: https://chromium-review.googlesource.com/269131
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
diff --git a/src/compiler/translator/OutputGLSLBase.h b/src/compiler/translator/OutputGLSLBase.h
index 92c657b..1f3558e 100644
--- a/src/compiler/translator/OutputGLSLBase.h
+++ b/src/compiler/translator/OutputGLSLBase.h
@@ -35,7 +35,7 @@
     void writeVariableType(const TType &type);
     virtual bool writeVariablePrecision(TPrecision precision) = 0;
     void writeFunctionParameters(const TIntermSequence &args);
-    const ConstantUnion *writeConstantUnion(const TType &type, const ConstantUnion *pConstUnion);
+    const TConstantUnion *writeConstantUnion(const TType &type, const TConstantUnion *pConstUnion);
     void writeConstructorTriplet(Visit visit, const TType &type, const char *constructorBaseType);
     TString getTypeName(const TType &type);