Module Debugging: Defer the emission of anonymous tag decls

until we are visiting their declcontext.

This fixes a regression introduced in r256962:
When building debug info for a typdef'd anonymous tag type, we would be
visiting the inner anonymous type first thus creating a "typedef changes
linkage of anonymous type, but linkage was already computed" error.

rdar://problem/24199640

llvm-svn: 258152