Improve some unreachable-after-loop code (facebook#13764)

Summary:
in log_reader.cc.
* `for (;;)` (with no matching break inside) should be more structurally recognizable to compilers as unreachable after compared to `while (true)` which compilers can treat as conditional for warning/error purposes because `true` might have come from a macro, etc.
* Comment the `break` statements to indicate they are for the `switch` (not the `for`)
* No code or annotation is apparently needed for the unreachable end of the non-void function, so just a comment

Pull Request resolved: facebook#13764

Test Plan: CI

Reviewed By: archang19

Differential Revision: D78135493

Pulled By: pdillinger

fbshipit-source-id: e313435a846a6e15346acf40404f755be98ab09a

Author: pdillinger

db/log_reader.cc

@@ -95,7 +95,7 @@ bool Reader::ReadRecord(Slice* record, std::string* scratch,
   uint64_t prospective_record_offset = 0;
 
   Slice fragment;
-  while (true) {
+  for (;;) {
     uint64_t physical_record_offset = end_of_buffer_offset_ - buffer_.size();
     size_t drop_size = 0;
     const uint8_t record_type =
@@ -140,7 +140,7 @@ bool Reader::ReadRecord(Slice* record, std::string* scratch,
         prospective_record_offset = physical_record_offset;
         scratch->assign(fragment.data(), fragment.size());
         in_fragmented_record = true;
-        break;
+        break;  // switch
 
       case kMiddleType:
       case kRecyclableMiddleType:
@@ -153,7 +153,7 @@ bool Reader::ReadRecord(Slice* record, std::string* scratch,
           }
           scratch->append(fragment.data(), fragment.size());
         }
-        break;
+        break;  // switch
 
       case kLastType:
       case kRecyclableLastType:
@@ -171,7 +171,7 @@ bool Reader::ReadRecord(Slice* record, std::string* scratch,
           first_record_read_ = true;
           return true;
         }
-        break;
+        break;  // switch
 
       case kSetCompressionType: {
         if (compression_type_record_read_) {
@@ -193,7 +193,7 @@ bool Reader::ReadRecord(Slice* record, std::string* scratch,
         } else {
           InitCompression(compression_record);
         }
-        break;
+        break;  // switch
       }
       case kPredecessorWALInfoType:
       case kRecyclePredecessorWALInfoType: {
@@ -210,7 +210,7 @@ bool Reader::ReadRecord(Slice* record, std::string* scratch,
           MaybeVerifyPredecessorWALInfo(wal_recovery_mode, fragment,
                                         recorded_predecessor_wal_info);
         }
-        break;
+        break;  // switch
       }
       case kUserDefinedTimestampSizeType:
       case kRecyclableUserDefinedTimestampSizeType: {
@@ -235,7 +235,7 @@ bool Reader::ReadRecord(Slice* record, std::string* scratch,
             ReportCorruption(fragment.size(), s.getState());
           }
         }
-        break;
+        break;  // switch
       }
 
       case kBadHeader:
@@ -304,7 +304,7 @@ bool Reader::ReadRecord(Slice* record, std::string* scratch,
           in_fragmented_record = false;
           scratch->clear();
         }
-        break;
+        break;  // switch
 
       case kBadRecordLen:
         if (eof_) {
@@ -337,7 +337,7 @@ bool Reader::ReadRecord(Slice* record, std::string* scratch,
           in_fragmented_record = false;
           scratch->clear();
         }
-        break;
+        break;  // switch
 
       default: {
         if ((record_type & kRecordTypeSafeIgnoreMask) == 0) {
@@ -349,11 +349,11 @@ bool Reader::ReadRecord(Slice* record, std::string* scratch,
         }
         in_fragmented_record = false;
         scratch->clear();
-        break;
+        break;  // switch
       }
     }
   }
-  return false;
+  // unreachable
 }
 
 void Reader::MaybeVerifyPredecessorWALInfo(