[clang-tidy] Adding Fuchsia checker for trailing returns

Adds a check to the Fuchsia module to warn if a function has a trailing
return.

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

Differential Revision: https://reviews.llvm.org/D42116

llvm-svn: 322759