When diagnosing C++ [temp.expl.spec]p3 in C++98/03 mode, downgrade the

error to a warning if we're in a case that would be allowed in
C++0x. This "fixes" PR8084 by making Clang accept more code than GCC
and (non-strict) EDG do. 

Also, add the missing test case for the C++0x semantics, which should
have been in r113717.

llvm-svn: 113718