Suppress implicit-conversion warnings seen with newer clang versions.

We were assigning values near 255 through "char *" pointers.  On machines
where char is signed, that's not entirely kosher, and it's reasonable
for compilers to warn about it.

A better solution would be to change the pointer type to "unsigned char *",
but that would be vastly more invasive.  For the moment, let's just apply
this simple backpatchable solution.

Aleksander Alekseev

Discussion: https://postgr.es/m/20170220141239.GD12278@e733.localdomain
Discussion: https://postgr.es/m/2839.1490714708@sss.pgh.pa.us

Author: tglsfdc

src/backend/access/transam/xlog.c

@@ -4878,7 +4878,7 @@ BootStrapXLOG(void)
 	record->xl_rmid = RM_XLOG_ID;
 	recptr += SizeOfXLogRecord;
 	/* fill the XLogRecordDataHeaderShort struct */
-	*(recptr++) = XLR_BLOCK_ID_DATA_SHORT;
+	*(recptr++) = (char) XLR_BLOCK_ID_DATA_SHORT;
 	*(recptr++) = sizeof(checkPoint);
 	memcpy(recptr, &checkPoint, sizeof(checkPoint));
 	recptr += sizeof(checkPoint);

src/backend/access/transam/xloginsert.c

@@ -703,7 +703,7 @@ XLogRecordAssemble(RmgrId rmid, uint8 info,
 	/* followed by the record's origin, if any */
 	if (include_origin && replorigin_session_origin != InvalidRepOriginId)
 	{
-		*(scratch++) = XLR_BLOCK_ID_ORIGIN;
+		*(scratch++) = (char) XLR_BLOCK_ID_ORIGIN;
 		memcpy(scratch, &replorigin_session_origin, sizeof(replorigin_session_origin));
 		scratch += sizeof(replorigin_session_origin);
 	}
@@ -713,13 +713,13 @@ XLogRecordAssemble(RmgrId rmid, uint8 info,
 	{
 		if (mainrdata_len > 255)
 		{
-			*(scratch++) = XLR_BLOCK_ID_DATA_LONG;
+			*(scratch++) = (char) XLR_BLOCK_ID_DATA_LONG;
 			memcpy(scratch, &mainrdata_len, sizeof(uint32));
 			scratch += sizeof(uint32);
 		}
 		else
 		{
-			*(scratch++) = XLR_BLOCK_ID_DATA_SHORT;
+			*(scratch++) = (char) XLR_BLOCK_ID_DATA_SHORT;
 			*(scratch++) = (uint8) mainrdata_len;
 		}
 		rdt_datas_last->next = mainrdata_head;

src/bin/pg_resetxlog/pg_resetxlog.c

@@ -1100,7 +1100,7 @@ WriteEmptyXLOG(void)
 	record->xl_rmid = RM_XLOG_ID;
 
 	recptr += SizeOfXLogRecord;
-	*(recptr++) = XLR_BLOCK_ID_DATA_SHORT;
+	*(recptr++) = (char) XLR_BLOCK_ID_DATA_SHORT;
 	*(recptr++) = sizeof(CheckPoint);
 	memcpy(recptr, &ControlFile.checkPointCopy,
 		   sizeof(CheckPoint));