Changed the serialization of IdentifierTable to only serialize out entries

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@43975 91177308-0d34-0410-b5e6-96231b3b80d8