C++ doesn't really use "namespaces" for different kinds of names the same

C++ doesn't really use "namespaces" for different kinds of names the same
way that C does.  Among other differences, elaborated type specifiers
are defined to skip "non-types", which, as you might imagine, does not
include typedefs.  Rework our use of IDNS masks to capture the semantics
of different kinds of declarations better, and remove most current lookup
filters.  Removing the last remaining filter is more complicated and will
happen in a separate patch.

Fixes PR 6885 as well some spectrum of unfiled bugs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@102164 91177308-0d34-0410-b5e6-96231b3b80d8