[NFC] [Serialization] Extract logics to write decls and types into a standalone function

This patch extract logics in ASTWriter::WriteASTCore about writing decls
and types into a standalone function. The WriteASTCore function is
pretty long and hard to read. It should be helpful for readability to extract the common
logics into a standalone function.

This is also helpful for further changes e.g., removing unreachable
declarations.