Try to plug some memory leaks...

Try to plug some memory leaks...

1) Sema::ParseAST now constructs a TranslationUnit object to own the top-level Decls, which releases the top-level Decls upon exiting ParseAST.

2) Bug fix: TranslationUnit::~TranslationUnit handles the case where a Decl is added more than once as a top-level Decl.

3) Decl::Destroy is now a virtual method, obviating the need for a special dispatch based on DeclKind.

3) FunctionDecl::Destroy now releases its Body using its Destroy method.

4) Added Stmt::Destroy and Stmt::DestroyChildren, which recursively delete the child ASTs of a Stmt and call their dstors.  We may need to special case dstor/Destroy methods for particular Stmt subclasses that own other dynamically allocated objects besides AST nodes.

5) REGRESSION: We temporarily are not deallocating attributes; a FIXME is provided.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@51286 91177308-0d34-0410-b5e6-96231b3b80d8