This allows use of a "default" expression that doesn't slavishly
match the data column's type.  Formerly you got something like
"function lag(numeric, integer, integer) does not exist", which
is not just unhelpful but actively misleading.

The SQL spec suggests that the default should be coerced to the data
column's type, but this implementation instead chooses the common
supertype, which seems at least as reasonable.

(Note: I took the opportunity to run "make reformat-dat-files" on
pg_proc.dat, so this commit includes some cosmetic changes to
recently-added entries that aren't related to lead/lag.)

Vik Fearing

Discussion: https://postgr.es/m/77675130-89da-dab1-51dd-492c93dcf5d1@postgresfriends.org