BSLS_IDENT_PRAGMA_ONCE for MSVC

MSVC supports '__pragma(once)', so use that for 'BSLS_IDENT_PRAGMA_ONCE'.

Signed-Off-By: Daryl Haresign <dharesign@bloomberg.net>
[APPROVAL] INTERNAL review/bslh_hash_osx_failure-merge-base-drqs-58605182 to master

Base commit: 3995e35707d2ef00248911e6a2b528f77796134e
[APPROVAL] review/add-MSVC-to-BSLS_IDENT_PRAGMA_ONCE-drqs-59136351 to master