[clangd] IncludeClenaer: Don't mark forward declarations of a class if it's...

[clangd] IncludeClenaer: Don't mark forward declarations of a class if it's declared in the main file

This will mark more headers that are unrelated to used symbol but contain its
forawrd declaration. E.g. the following are examples of headers forward
declaring `llvm::StringRef`:

- clang/include/clang/Basic/Cuda.h
- llvm/include/llvm/Support/SHA256.h
- llvm/include/llvm/Support/TrigramIndex.h
- llvm/include/llvm/Support/RandomNumberGenerator.
- ... and more (~50 in total)

This patch is a reduced version of D112707 which was controversial.

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D114864