[NewPM] Set -enable-npm-optnone to true by default

This makes the NPM skip not required passes on functions marked optnone.

If this causes a pass that should be required but has not been marked
required to be skipped, add
`static bool isRequired() { return true; }`
to the pass class. AlwaysInlinerPass is an example.

clang/test/CodeGen/O0-no-skipped-passes.c is useful for checking that
no passes are skipped under -O0.

The -enable-npm-optnone option will be removed once this has been stable
for long enough without issues.

Reviewed By: ychen, asbirlea

Differential Revision: https://reviews.llvm.org/D87869