This transformation is correct for a builtin call to 'free(p)', but not
for 'operator delete(p)'. There is no guarantee that a user replacement
'operator delete' has no effect when called on a null pointer.
However, the principle behind the transformation *is* correct, and can
be applied more broadly: a 'delete p' expression is permitted to
unconditionally call 'operator delete(p)'. So do that in Clang under
-Oz where possible. We do this whether or not 'p' has trivial
destruction, since the destruction might turn out to be trivial after
inlining, and even for a class-specific (but non-virtual,
non-destroying, non-array) 'operator delete'.