[clang] [clang-tools-extra] [lldb] [llvm] [clang][NFC] Inline Frontend/FrontendDiagnostic.h -> Basic/DiagnosticFrontend.h (PR #162883)

[Jordan Rupprecht via llvm-commits]

rupprecht wrote:

This is still something I'd like to land. It's not urgent, but it's been a month :)

@AaronBallman, does this change seem fine to you? Or is there someone else from the Clang side that could take a quick look?

https://github.com/llvm/llvm-project/pull/162883