Explicit representation of ASTs for all the functions in a compile unit.

Added AST and Function classes.
Now, we parse the whole file and build up the AST for all of the
  functions in the Module before we emit IR for the functions (vs. before,
  when we generated IR along the way as we parsed the source file.)

ispc.h

@@ -90,8 +90,8 @@ class Declarator;
 class FunctionEmitContext;
 class Expr;
 class ExprList;
+class Function;
 class FunctionType;
-class GatherBuffer;
 class Module;
 class Stmt;
 class Symbol;
@@ -124,37 +124,6 @@ struct SourcePos {
 };
 
 
-/** @brief Abstract base class for nodes in the abstract syntax tree (AST).
-
-    This class defines a basic interface that all abstract syntax tree
-    (AST) nodes must implement.  The base classes for both expressions
-    (Expr) and statements (Stmt) inherit from this class.
-*/
-class ASTNode {
-public:
-    ASTNode(SourcePos p) : pos(p) { }
-    virtual ~ASTNode();
-
-    /** The Optimize() method should perform any appropriate early-stage
-        optimizations on the node (e.g. constant folding).  The caller
-        should use the returned ASTNode * in place of the original node.
-        This method may return NULL if an error is encountered during
-        optimization. */
-    virtual ASTNode *Optimize() = 0;
-
-    /** Type checking should be performed by the node when this method is
-        called.  In the event of an error, a NULL value may be returned.
-        As with ASTNode::Optimize(), the caller should store the returned
-        pointer in place of the original ASTNode *. */
-    virtual ASTNode *TypeCheck() = 0;
-
-    virtual int EstimateCost() const = 0;
-
-    /** All AST nodes must track the file position where they are
-        defined. */
-    const SourcePos pos;
-};
-
 /** @brief Structure that defines a compilation target 
 
     This structure defines a compilation target for the ispc compiler.