pgsql: Back-patch addition of pg_wchar-to-multibyte conversion function

Back-patch addition of pg_wchar-to-multibyte conversion functionality.

Back-patch of commits 72dd6291f216440f6bb61a8733729a37c7e3b2d2,
f6a05fd973a102f7e66c491d3f854864b8d24844, and
60e9c224a197aa37abb1aa3aefa3aad42da61f7f.

This is needed to support fixing the regex prefix extraction bug in
back branches.

Branch
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REL8_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/6b304087a752d0f776aba09e93f9203f2039a766

Modified Files
--------------
src/backend/utils/mb/mbutils.c |   22 +++
src/backend/utils/mb/wchar.c   |  287 ++++++++++++++++++++++++++++++++++------
src/include/mb/pg_wchar.h      |   32 ++++-
3 files changed, 293 insertions(+), 48 deletions(-)