Significantly reduce the tendrils of DeclSpecs/Declarator/Declaration code

The stuff in decl.h/decl.cpp is messy, largely due to its close mapping
to C-style variable declarations.  This checkin has updated code throughout
all of the declaration statement, variable, and function code that operates
on symbols and types directly.  Thus, Decl* related stuff is now localized
to decl.h/decl.cpp and the parser.

Issue #13.

ispc.h

@@ -84,9 +84,6 @@ namespace llvm {
 
 class ArrayType;
 class AtomicType;
-class DeclSpecs;
-class Declaration;
-class Declarator;
 class FunctionEmitContext;
 class Expr;
 class ExprList;
@@ -97,6 +94,7 @@ class Stmt;
 class Symbol;
 class SymbolTable;
 class Type;
+struct VariableDeclaration;
 
 /** @brief Representation of a range of positions in a source file.