[TransformWarning] Do not warn missed transformations in optnone functions.

Optimization transformations are intentionally disabled by the 'optnone'
function attribute. Therefore do not warn if transformation metadata is
still present.

Using the legacy pass manager structure, the `skipFunction` method takes
care for the optnone attribute (already called before this patch). For
the new pass manager, there is no equivalent, so we check for the
'optnone' attribute manually.

Differential Revision: https://reviews.llvm.org/D55690

llvm-svn: 349184