pgsql: Move codes for pg_backend_memory_contexts from mmgr/mcxt.c to ad

Move codes for pg_backend_memory_contexts from mmgr/mcxt.c to adt/mcxtfuncs.c.

Previously the codes for pg_backend_memory_contexts were in
src/backend/utils/mmgr/mcxt.c. This commit moves them to
src/backend/utils/adt/mcxtfuncs.c so that mcxt.c basically includes
only the low-level interface for memory contexts.

Author: Atsushi Torikoshi
Reviewed-by: Michael Paquier, Fujii Masao
Discussion: https://postgr.es/m/20200819135545.GC19121@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/50db5964ee333bc148e0c8844ffafaf585c719c6

Modified Files
--------------
src/backend/utils/adt/Makefile | 1 +
src/backend/utils/adt/mcxtfuncs.c | 157 ++++++++++++++++++++++++++++++++++++++
src/backend/utils/mmgr/mcxt.c | 137 ---------------------------------
3 files changed, 158 insertions(+), 137 deletions(-)