pgsql: Hide ICU C++ APIs from pg_locale.h

Hide ICU C++ APIs from pg_locale.h

The cpluspluscheck script wraps our headers in `extern "C"`. This
disables name mangling, which is necessary for the C++ templates
in system ICU headers. cpluspluscheck thus fails when the build is
configured with ICU (the default). CI worked around this by disabling
ICU, but let's make it work so others can run the script.

We can specify we only want the C APIs by defining U_SHOW_CPLUSPLUS_API
to be 0 in pg_locale.h. Extensions that want the C++ APIs can include
ICU headers separately before including PostgreSQL headers.

ICU documentation:
https://github.com/unicode-org/icu/blob/main/docs/processes/release/tasks/healthy-code.md#test-icu4c-headers

Suggested-by: Andres Freund <andres(at)anarazel(dot)de>
Reviewed-by: Peter Eisentraut <peter(at)eisentraut(dot)org>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/20220323002024.f2g6tivduzrktgfa%40alap3.anarazel.de
Discussion: https://postgr.es/m/CANWCAZbgiaz1_0-F4SD%2B%3D-e9onwAnQdBGJbhg94EqUu4Gb7WyA%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/ed26c4e25a444fcdd8a9120d4fe4b5a08d2b262b

Modified Files
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.cirrus.tasks.yml | 3 ---
src/include/utils/pg_locale.h | 3 +++
2 files changed, 3 insertions(+), 3 deletions(-)