[lldb] Initialize NameSearchContext::m_namespace_map in constructor

This member is for some reason initialized in ClangASTSource::FindExternalVisibleDecls
so all other functions using this member dereference a nullptr unless we
call this function before that. Let's just initialize this in the constructor.

This should be NFC as the only side effect is that we don't reset the namespace map
when calling ClangASTSource::FindExternalVisibleDecls multiple times (and we never
call this function multiple times for one NameSearchContext from what I can see).