In DeclContext::LoadLexicalDeclsFromExternalStorage don't clear out

the fields if they are already loaded, just ignore them when we are building
the chain in BuildDeclChain.

This fixes an lldb issue where fields were removed and not getting re-added
because lldb is based on ASTImporter adding decls to DeclContext and fields
were already added before by the ASTImporter.

We should really simplify the interaction between DeclContext <-> lldb
going forward..

rdar://10246067

llvm-svn: 141418