[C++20] [Modules] [Reduced BMI] Write Special Decl Lazily

ASTWrite would write some special records for the consumer of the AST
can get the information easily. This is fine. But currently all the
special records are written eagerly, which is conflicting with reduced
BMI.

In reduced BMI, we hope to write things as less as possible. It is not a
goal for reduced BMI to retain all the informations. So in this patch we
won't record the special declarations eagerly before we starting write.
But only writing the sepcial records after we writes decls and types,
then only the reached declarations will be collected.