Add `Dump` functions for `ClangDecl`/`ClangDeclSignature`, and add
`MakeClangDeclId`/`MakeClangDeclSignatureId` and add those to
`lldbinit.py`.
Also change the label for `ClangDeclId` and `ClangDeclSignatureId` to
not have an "_id" suffix, to make them consistent with other ID types.