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.cpp

@@ -311,12 +311,6 @@ Globals::Globals() {
 #endif
 }
 
-///////////////////////////////////////////////////////////////////////////
-// ASTNode
-
-ASTNode::~ASTNode() {
-}
-
 ///////////////////////////////////////////////////////////////////////////
 // SourcePos