Do cleanup on explorer fuzzing infrastructure. (#2790)

I'm partly doing this because the current setup would be difficult to share with the toolchain. e.g., ProtoToCarbon isn't explorer-specific, but the only way to run it via CLI is the explorer's fuzzverter. I want a separate tool.

This change:

- Adds a //common/fuzzing:proto_to_carbon tool.
  - The rest of fuzzverter is now just //explorer/fuzzing:ast_to_proto.
  - The change simplifies overall handling and removes a LLVM CLI dependency.
- Stops allowing unknown fields in the proto.
  - This has mostly led to forgetting to remove fuzzer inputs that were for removed features.
- Moves more non-explorer-specific bits to //common/fuzzing.
- Cleans up remaining pieces in //explorer/fuzzing
  - Merges the //explorer/fuzzing proto tests, which deduplicates AstToString copies.
    - These tests also had duplicate dependencies, etc -- and all complete in ~6s.
  - Updates and fixes regen_corpus which was previously broken by other changes.
  - Updates the README to reflect changes.
- Removes obsolete proto-fuzzer build configuration (AFAICT this is no longer needed).

explorer/fuzzing/ast_to_proto.cpp

@@ -843,15 +843,15 @@ static auto DeclarationToProto(const Declaration& declaration)
   return declaration_proto;
 }
 
-auto AstToProto(const AST& ast) -> Fuzzing::CompilationUnit {
-  Fuzzing::CompilationUnit compilation_unit;
-  *compilation_unit.mutable_package_statement() =
-      LibraryNameToProto(ast.package);
-  compilation_unit.set_is_api(ast.is_api);
+auto AstToProto(const AST& ast) -> Fuzzing::Carbon {
+  Fuzzing::Carbon carbon;
+  auto* unit = carbon.mutable_compilation_unit();
+  *unit->mutable_package_statement() = LibraryNameToProto(ast.package);
+  unit->set_is_api(ast.is_api);
   for (const Declaration* declaration : ast.declarations) {
-    *compilation_unit.add_declarations() = DeclarationToProto(*declaration);
+    *unit->add_declarations() = DeclarationToProto(*declaration);
   }
-  return compilation_unit;
+  return carbon;
 }
 
 }  // namespace Carbon