Render anonymous entities as '(anonymous <thing>)' (and lambdas as '(lambda at ... )')

For namespaces, this is consistent with mangling and GCC's debug info
behavior. For structs, GCC uses <anonymous struct> but we prefer
consistency between all anonymous entities but don't want to confuse
them with template arguments, etc, so we'll just go with parens in all
cases.

llvm-svn: 205398