This simplifies the core benefit of -flimit-debug-info by taking a more
systematic approach to avoid emitting debug info definitions for types
that only require declarations. The previous ad-hoc approach (3 cases
removed in this patch) had many holes.
The general approach (adding a bit to TagDecl and callback through
ASTConsumer) has been discussed with Richard Smith - though always open
to revision.
llvm-svn: 186262