[clang-tidy] new cppcoreguidelines-slicing

Flags slicing of member variables or vtable. See:

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es63-dont-slice
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c145-access-polymorphic-objects-through-pointers-and-references

Differential revision:
http://reviews.llvm.org/D21974

llvm-svn: 276408