Don't correct typos in Sema::BuildCXXNestedNameSpecifier with -fms-extensions

When -fms-extensions is enabled, the typo correction was being called here on
non-error paths (as in test/SemaTemplate/lookup-dependent-bases.cpp) and correct
compilation depended on Sema::CorrectTypo not finding a viable candidate.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@191046 91177308-0d34-0410-b5e6-96231b3b80d8