This header is semi-private, being used only in files related to
raw parsing, so move to the backend directory where those files
live. This allows removal of Makefile rules that symlink gram.h to
src/include/parser, since gramparse.h can now include gram.h from
within the same directory. This has the side-effect of no longer
installing gram.h and gramparse.h, but there doesn't seem to be a
good reason to continue doing so.
Per suggestion from Andres Freund and Peter Eisentraut
Discussion: https://www.postgresql.org/message-id/20220904181759.px6uosll6zbxcum5%40awork3.anarazel.de
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