Generate fwd decls from nested cling instance
Upstream Merge
Workarounds for getting STL fwd decls working:
Iterate extern C blocks instead of calling VisitDeclContext
Check if :: is a substring for hasNestedNameSpecifier
Discard old solution for not printing classes and templates twice.
Upstream Merge
Improvements for STL forward declarations.

Iterate extern C blocks instead of calling VisitDeclContext
Check if :: is a substring for hasNestedNameSpecifier
Discard old solution for not printing classes and templates twice.