Improve debugability of clang decls (#7945)

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.

scripts/lldbinit.py

@@ -82,6 +82,8 @@ Example usage:
 
     # The set of "Make" functions in dump.cpp.
     id_types = {
+        "clang_decl": "SemIR::MakeClangDeclId",
+        "clang_decl_signature": "SemIR::MakeClangDeclSignatureId",
         "class": "SemIR::MakeClassId",
         "constant": "SemIR::MakeConstantId",
         "constraint": "SemIR::MakeNamedConstraintId",