Fix 64-bit shifting in dynahash.c

Fix 64-bit shifting in dynahash.c

The switch from long to int64 in commit 13b935cd521 was incomplete.
It was shifting the constant 1L, which is not always 64 bit.  Fix by
using an explicit int64 constant.

MSVC warning:

../src/backend/utils/hash/dynahash.c(1767): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?)

Also add the corresponding warning to the standard warning set on
MSVC, to help catch similar issues in the future.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/1142ad86-e475-41b3-aeee-c6ad913064fa%40eisentraut.org