Re-commit r247216: "Fix Clang-tidy misc-use-override warnings, other minor fixes"

Re-commit r247216: "Fix Clang-tidy misc-use-override warnings, other minor fixes"

Except the changes that defined virtual destructors as =default, because that
ran into problems with GCC 4.7 and overriding methods that weren't noexcept.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247298 91177308-0d34-0410-b5e6-96231b3b80d8