[X86] Always define the tzcnt intrinsics even when _MSC_VER is defined.

These intrinsics use llvm.cttz intrinsics so are always available
even without the bmi feature. We already don't check for the bmi
feature on the intrinsics themselves. But we were blocking the
include of the header file with _MSC_VER unless BMI was enabled
on the command line.

Fixes PR30506.

llvm-svn: 374516