[DWARFASTParserClang] Complete external record types before using them as a decl context.

Summary:
When in a gmodules-like debugging scenario, you can have a parent decl context
that gets imported from an external AST. When this happens, we must be careful
to complete this type before adding children to it, otherwise it sometimes
results in a crash.

Reviewers: clayborg, jingham

Subscribers: aprantl, JDevlieghere, lldb-commits

Differential Revision: https://reviews.llvm.org/D43592

llvm-svn: 327750