Correctly look up declarations in inline namespaces

Summary:
This patch marks the inline namespaces from DWARF as inline and also ensures that looking
up declarations now follows the lookup rules for inline namespaces.

Reviewers: aprantl, shafik, serge-sans-paille

Reviewed By: aprantl

Subscribers: eraman, jdoerfert, lldb-commits

Tags: #c_modules_in_lldb, #lldb

Differential Revision: https://reviews.llvm.org/D59198

llvm-svn: 355897