[clang-tidy] Exclude forward decls from fuchsia-multiple-inheritance

Addresses b39770.

Differential Revision: https://reviews.llvm.org/D64813

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@366354 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed