Audit uses of Sema::LookupSingleName for those lookups that are

intended for redeclarations, fixing those that need it. Fixes PR6831.

This uncovered an issue where the C++ type-specifier-seq parsing logic
would try to perform name lookup on an identifier after it already had
a type-specifier, which could also lead to spurious ambiguity errors
(as in PR6831, but with a different test case).

llvm-svn: 101419