Added method "HandleTopLevelDeclaration" to ASTConsumer. This will eventually

be the new hook that ASTStreamer calls to feed top-level Decls to
ASTConsumers.

The difference between "HandleTopLevelDeclaration" and "HandleTopLevelDecl" is
that "HandleTopLevelDecl" is currently called by ASTStreamer for every
top-level declaration, including those that appear within a Decl chain. Using
the new interface, ASTStreamer would only call HandleTopLevelDeclaration for
Decls that appear that the beginning of a Decl chain (i.e., a group of related
decls).

To preserve the behavior that all subclasses of ASTConsumer currently expect,
the default implementation of HandleTopLevelDeclaration simply calls
HandleTopLevelDecl, and for decl chains it calls HandleTopLevelDecl for each
Decl* in a chain of Decls.

The advantage of this interface is that some subclasses of ASTConsumer only
really want the Decl chain, and not each individual Decl passed to them. This
extra level of indirection allows subclasses to override the default behavior
if they so desire.

llvm-svn: 44444