pgsql: Add MSVC support for pg_leftmost_one_pos32() and friends

Add MSVC support for pg_leftmost_one_pos32() and friends

To allow testing for general support for fast bitscan intrinsics,
add symbols HAVE_BITSCAN_REVERSE and HAVE_BITSCAN_FORWARD.

Also do related cleanup in AllocSetFreeIndex(): Previously, we
tested for HAVE__BUILTIN_CLZ and copied the relevant internals of
pg_leftmost_one_pos32(), with a special fallback that does less
work than the general fallback for that function. Now that we have
a more general test, we just call pg_leftmost_one_pos32() directly
for platforms with intrinsic support. On gcc at least, there is no
difference in the binary for non-assert builds.

Discussion: https://www.postgresql.org/message-id/CAFBsxsEPc%2BFnX_0vmmQ5DHv60sk4rL_RZJ%2BMD6ei%3D76L0kFMvA%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/67731974644ce77129d282434065c3f0be04be92

Modified Files
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src/backend/utils/mmgr/aset.c | 6 ++--
src/include/port/pg_bitutils.h | 75 +++++++++++++++++++++++++++++++++++++-----
2 files changed, 70 insertions(+), 11 deletions(-)