For some LLVM-as-library uses it is convenient to create a
subclass of TargetMachine which "forwards" all operations to an
existing internal TargetMachine member variable. In the usage context the
specific-machine class derived from TargetMachine is not visible,
only a reference to the generic base class TargetMachine. Although
getSubtargetImpl() is public in specific-machine classes derived from
TargetMachine, the TargetMachine class unfortunately has
getSubtargetImpl() protected (and accessing non-const members makes
abusing getSubtarget() unsuitable). Making it public in the base class
allows this forwarding pattern.