In ASTWriter::WriteDeclContextVisibleBlock, don't write empty lookups.

Empty lookups can occur in the DeclContext map when we are chaining PCHs, where
the empty lookup indicates that we already looked in ExternalASTSource.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@138816 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed