[clang-tidy] Ignore forward declarations without definitions in the same...

[clang-tidy] Ignore forward declarations without definitions in the same translation unit in readability-identifier-naming

Summary: This change ensures that forward declarations of classes are not considered for identifier naming checks within a translation unit.

Reviewers: alexfh, aaron.ballman

Subscribers: mgehre

Differential Revision: https://reviews.llvm.org/D22571

llvm-svn: 285907