Allow redefinitions of typedef-names within class scope when the type

Allow redefinitions of typedef-names within class scope when the type
they redefine is a class-name but not a typedef-name, per C++0x
[dcl.typedef]p4. The code in the test was valid C++98 and is valid
C++0x, but an unintended consequence of DR56 made it ill-formed in
C++03 (which we were luck enough to implement). Fixes PR5455.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@93188 91177308-0d34-0410-b5e6-96231b3b80d8