protoc-gen-c/compat.h: Add `compat::StringView` type

protobuf >= 30.x replaces `const std::string&` in various APIs with
its own string view type that may actually be a `absl::string_view`.
Introduce our own `compat::StringView` type that we can use instead
of `const std::string&` in order to support compiling across multiple
protobuf versions.

protoc-gen-c/compat.h

@@ -28,6 +28,8 @@
 #ifndef PROTOBUF_C_PROTOC_GEN_C_COMPAT_H__
 #define PROTOBUF_C_PROTOC_GEN_C_COMPAT_H__
 
+#include <string>
+
 #if GOOGLE_PROTOBUF_VERSION >= 4022000
 # define GOOGLE_ARRAYSIZE	ABSL_ARRAYSIZE
 # define GOOGLE_CHECK_EQ	ABSL_CHECK_EQ
@@ -39,6 +41,12 @@ namespace protobuf_c {
 
 namespace compat {
 
+#if GOOGLE_PROTOBUF_VERSION >= 6030000
+typedef google::protobuf::internal::DescriptorStringView StringView;
+#else
+typedef const std::string& StringView;
+#endif
+
 }  // namespace compat
 
 }  // namespace protobuf_c