Limit our MSVC compat hack for nested names from dependent bases

Previously, any undeclared unqualified id starting a nested name
specifier in a dependent context would have its lookup retried during
template instantiation.  Now we limit that retry hack to methods of a
class with dependent bases.  Free function templates in particular are
no longer affected by this hack.

Also, diagnose this as a Microsoft extension. This has the downside that
template authors may see this warning *and* an error during
instantiation time about this identifier. Fixing that will probably
require formalizing some kind of "delayed" identifier, instead of our
ad-hoc solutions of forming dependent AST nodes when lookup fails.

Based on a patch by Kim Gräsman!

Differential Revision: http://reviews.llvm.org/D4854