KVM: x86: fix clang build

Clang resolves __builtin_constant_p() to false even if the expression is
constant in the end.  The only purpose of that expression was to
differentiate a case where the following expression couldn't be checked
at compile-time, so we can just remove the check.

Clang handles the following two correctly.  Turn it into BUG_ON if there
are any more problems with this.

Fixes: d6321d49 ("KVM: x86: generalize guest_cpuid_has_ helpers")
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/cpuid.h

@@ -59,7 +59,6 @@ static __always_inline struct cpuid_reg x86_feature_cpuid(unsigned x86_feature)
 {
 	unsigned x86_leaf = x86_feature / 32;
 
-	BUILD_BUG_ON(!__builtin_constant_p(x86_leaf));
 	BUILD_BUG_ON(x86_leaf >= ARRAY_SIZE(reverse_cpuid));
 	BUILD_BUG_ON(reverse_cpuid[x86_leaf].function == 0);