ASTIdentifierNode now inherits from ASTExpressionNode directly

[nedyoNK]

Addressing this PR to resolve issue #6.

Changes made to simplify the code:
ASTExpressionNode is moved from ASTTypes.h into ASTExpression.h. The constructors and methods of ASTExpressionNode that require a complete ASTIdentifierNode type are placed in ASTExpressionNodeImpl.h, which is included at the bottom of ASTExpression.h after ASTIdentifierNode is fully defined.

ASTQualifiers.h had its circular includes removed; the three qualifier constructors taking const ASTIdentifierNode& are moved to ASTExpressionNodeImpl.h.

ASTIdentifierNode now inherits ASTExpressionNode instead of ASTExpression. The EXP member field and the SetExpression/GetExpression/HasExpression methods are removed from ASTIdentifierNode since the relationship is now expressed via the inheritance hierarchy.

GetIndexIdentifier() in ASTIdentifierNode is renamed to GetIndexIdentifierName() to avoid a return-type conflict with ASTExpressionNode::GetIndexIdentifier(). Callers in ASTBuilder.cpp are updated correspondingly."

If this solution is viable feel free to merge it. :)

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