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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205398 91177308-0d34-0410-b5e6-96231b3b80d8