One more UserIDResolver fix

The intention in r355323 has been to implement a no-op resolver in the
HostInfoBase class, which will then be shadowed a an implementation in
the HostInfoPosix class. However, I add the shadowing declaration in
HostInfoPosix.h, and instead had implemented the HostInfoBase function
in HostInfoPosix.cpp. This has lead to undefined symbols on windows, and
a subsequent implementation of a no-op resolver in HostInfoWindows
(r355329).

Since now there is no point on having a no-op resolver in the base
class, I just remove the base declaration altogether, and have
HostInfoPosix implement the (newly-declared) HostInfoPosix version of
that function.

llvm-svn: 355398