Changed the serialization of IdentifierTable to only serialize out entries

that are referenced in the ASTs. This assumes that we serialize out the
decls/stmts first, and use the pointer-tracking logic in the Serializer to
determine if an IdentifierInfo (or its string key) is ever referenced.

This is a significant space optimization for serialized ASTs.

Consider the following program:

void foo(int x,int y) {
  return x > y ? x : y+1;
}

Here are the sizes of the files for the serialized ASTs:

        Full IdentifierTable: 23676 bytes
 Only-referenced Identifiers:   304 bytes.

For this simple program, this is a 77% reduction in the file size of the
serialized ASTs.

llvm-svn: 43975