[clangd] IncludeCleaner: Do not require forward declarations of RecordDecls...

[clangd] IncludeCleaner: Do not require forward declarations of RecordDecls when definition is available

This makes IncludeCleaner more useful in the presense of a large number of
forward declarations. If the definition is already in the Translation Unit and
visible to the Main File, forward declarations have no effect.

The original patch D112707 was split in two: D114864 and this one.

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D114949