pgsql: Suppress implicit-conversion warnings seen with newer clangvers

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

Branch
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REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/8433e0b40e0d840b8daa80eb9788fafcf2dc5aab

Modified Files
--------------
src/backend/access/transam/xlog.c       | 2 +-
src/backend/access/transam/xloginsert.c | 6 +++---
src/bin/pg_resetxlog/pg_resetxlog.c     | 2 +-
3 files changed, 5 insertions(+), 5 deletions(-)