[clang-tidy] Adding Fuchsia checker for virtual inheritance

Adds a check to the Fuchsia module to warn if classes are defined
with virtual inheritance.

See https://fuchsia.googlesource.com/zircon/+/master/docs/cxx.md for
reference.

Differential Revision: https://reviews.llvm.org/D40813

llvm-svn: 320841