Fix some false-positives with cppcoreguidelines-pro-type-member-init. Handle...

Fix some false-positives with cppcoreguidelines-pro-type-member-init. Handle classes with default constructors that are defaulted or are not present in the AST.
Classes with virtual methods or virtual bases are not trivially default constructible, so their members and bases need to be initialized.

Patch by Malcolm Parsons.

llvm-svn: 283224