core: Make variable shadowing a compile-time error

Now that we have most of core free of shadowing, we can enable the
warning as an error to catch anything that may be remaining and also
eliminate this class of logic bug entirely.

src/core/hle/service/bcat/backend/boxcat.cpp

@@ -3,9 +3,18 @@
 // Refer to the license.txt file included.
 
 #include <fmt/ostream.h>
+
+#ifdef __GNUC__
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wshadow"
+#endif
 #include <httplib.h>
 #include <mbedtls/sha256.h>
 #include <nlohmann/json.hpp>
+#ifdef __GNUC__
+#pragma GCC diagnostic pop
+#endif
+
 #include "common/hex_util.h"
 #include "common/logging/backend.h"
 #include "common/logging/log.h"
@@ -178,8 +187,8 @@ bool VfsRawCopyDProgress(FileSys::VirtualDir src, FileSys::VirtualDir dest,
 
 class Boxcat::Client {
 public:
-    Client(std::string path, u64 title_id, u64 build_id)
-        : path(std::move(path)), title_id(title_id), build_id(build_id) {}
+    Client(std::string path_, u64 title_id_, u64 build_id_)
+        : path(std::move(path_)), title_id(title_id_), build_id(build_id_) {}
 
     DownloadResult DownloadDataZip() {
         return DownloadInternal(fmt::format(BOXCAT_PATHNAME_DATA, title_id), TIMEOUT_SECONDS,