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/fuzzer_util_test.cpp

@@ -9,6 +9,7 @@
 
 #include <fstream>
 
+#include "common/fuzzing/proto_to_carbon.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/raw_ostream.h"
 
@@ -41,38 +42,17 @@ TEST(FuzzerUtilTest, ParseAndExecute) {
       }
     })");
   ASSERT_TRUE(carbon_proto.ok());
-  const ErrorOr<int> result = ParseAndExecute(carbon_proto->compilation_unit());
+  const ErrorOr<int> result = ParseAndExecute(*carbon_proto);
   ASSERT_TRUE(result.ok()) << "Execution failed: " << result.error();
   EXPECT_EQ(*result, 0);
 }
 
 TEST(FuzzerUtilTest, GetRunfilesFile) {
-  EXPECT_THAT(*Internal::GetRunfilesFile("carbon/explorer/data/prelude.carbon"),
+  EXPECT_THAT(*GetRunfilesFile("carbon/explorer/data/prelude.carbon"),
               testing::EndsWith("/prelude.carbon"));
-  EXPECT_THAT(Internal::GetRunfilesFile("nonexistent-file").error().message(),
+  EXPECT_THAT(GetRunfilesFile("nonexistent-file").error().message(),
               testing::EndsWith("doesn't exist"));
 }
 
-TEST(FuzzerUtilTest, ParseCarbonTextProtoWithUnknownField) {
-  const ErrorOr<Fuzzing::Carbon> carbon_proto =
-      ParseCarbonTextProto(R"(
-    compilation_unit {
-      garbage: "value"
-      declarations {
-        choice {
-          name {
-            name: "Ch"
-          }
-        }
-      }
-    })",
-                           /*allow_unknown=*/true);
-  ASSERT_TRUE(carbon_proto.ok());
-  // No EqualsProto in gmock - https://github.com/google/googletest/issues/1761.
-  EXPECT_EQ(
-      carbon_proto->compilation_unit().declarations(0).choice().name().name(),
-      "Ch");
-}
-
 }  // namespace
 }  // namespace Carbon::Testing