Prevent unreachable when checking invalid multiversion decls.

CPUSpecifc/CPUDispatch call resolution assumed that all declarations
that would be passed are valid, however this was an invalid assumption.
This patch deals with those situations by making the valid version take
priority.  Note that the checked ordering is arbitrary, since both are
replaced by calls to the resolver later.

Change-Id: I7ff2ec88c55a721d51bc1f39ea1a1fe242b4e45f
llvm-svn: 350398