[SemaCXX] add -Woverride-init alias to -Winitializer-overrides

Summary:
https://bugs.llvm.org/show_bug.cgi?id=40251
https://github.com/ClangBuiltLinux/linux/issues/307

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: cfe-commits, nathanchance, srhines

Differential Revision: https://reviews.llvm.org/D56522

llvm-svn: 350877