[lldb] Rewrite/expand TestCppTypedef and document a lookup bug.

Just regrouping the checks for the same typedef together and also giving the
different typedefs unique names. We might want to have a second test with
identical names to see how LLDB handle the potential name conflict, but that
should be a separate test and not part of the main typedef test.

Also this test is actually unintentionally passing. LLDB can't lookup typedefs
in a struct/class scope, but in the test the check passes as the local variable
in the expression evaluation scope pulls in the typedef. I added a second check
that makes it clear that this is not working right now.