Enable -Wimplicit-fallthrough in proxygen/PACKAGE

Summary:
This diff enables the titular warning flag for the directory in question. Further details are in [this workplace post](https://fb.workplace.com/permalink.php?story_fbid=pfbid02XaWNiCVk69r1ghfvDVpujB8Hr9Y61uDvNakxiZFa2jwiPHscVdEQwCBHrmWZSyMRl&id=100051201402394).

This is a low-risk diff. There are **no run-time effects** and the diff has already been observed to compile locally. **If the code compiles, it works; test errors are spurious.**

If the diff does not pass, it will be closed automatically.

Reviewed By: dmm-fb

Differential Revision: D53545477

fbshipit-source-id: 9b4655fc6505e9f43f21251bbd47a84efaeca313

proxygen/external/http_parser/http_parser_cpp.cpp

@@ -530,7 +530,7 @@ parse_url_char(enum state s, const char ch, int strict_flag)
 #if __cplusplus
       [[fallthrough]];
 #else /* __cplusplus */
-    /* FALLTHROUGH */
+    __attribute__((fallthrough));
 #endif /* __cplusplus */
     case s_req_server_start:
     case s_req_server:
@@ -2252,7 +2252,7 @@ http_parse_host_char(enum http_host_state s, const char ch) {
 #if __cplusplus
       [[fallthrough]];
 #else /* __cplusplus */
-    /* FALLTHROUGH */
+    __attribute__((fallthrough));
 #endif /* __cplusplus */
     case s_http_host_v6_end:
       if (ch == ':') {
@@ -2269,7 +2269,7 @@ http_parse_host_char(enum http_host_state s, const char ch) {
 #if __cplusplus
       [[fallthrough]];
 #else /* __cplusplus */
-    /* FALLTHROUGH */
+    __attribute__((fallthrough));
 #endif /* __cplusplus */
     case s_http_host_v6_start:
       if (IS_HEX(ch) || ch == ':' || ch == '.') {
@@ -2412,7 +2412,7 @@ http_parser_parse_url_options(const char *buf, size_t buflen, int is_connect,
 #if __cplusplus
         [[fallthrough]];
 #else /* __cplusplus */
-      /* FALLTHROUGH */
+      __attribute__((fallthrough));
 #endif /* __cplusplus */
       case s_req_server:
         uf = UF_HOST;

proxygen/lib/http/experimental/RFC1867.cpp

@@ -80,7 +80,7 @@ std::unique_ptr<IOBuf> RFC1867Codec::onIngress(std::unique_ptr<IOBuf> data) {
         input_.trimStart(boundary_.length() - 1);
         bytesProcessed_ += boundary_.length() - 1;
         state_ = ParserState::HEADERS_START;
-        // fall through
+        [[fallthrough]];
 
       case ParserState::HEADERS_START: {
         if (input_.chainLength() < 3) {
@@ -117,7 +117,7 @@ std::unique_ptr<IOBuf> RFC1867Codec::onIngress(std::unique_ptr<IOBuf> data) {
         headerParser_.onIngress(*dummyBuf);
         CHECK(!parseError_);
         state_ = ParserState::HEADERS;
-        // fall through
+        [[fallthrough]];
 
       case ParserState::HEADERS:
         while (!parseError_ && input_.front() &&