jit: Remove -Wno-deprecated-declarations in 18+.

REL_18_STABLE and master have commit ee485912, so they always use the
newer LLVM opaque pointer functions.  Drop -Wno-deprecated-declarations
(commit a56e7b660) for code under jit/llvm in those branches, to catch
any new deprecation warnings that arrive in future version of LLVM.

Older branches continued to use functions marked deprecated in LLVM 14
and 15 (ie switched to the newer functions only for LLVM 16+), as a
precaution against unforeseen compatibility problems with bitcode
already shipped.  In those branches, the comment about warning
suppression is updated to explain that situation better.  In theory we
could suppress warnings only for LLVM 14 and 15 specifically, but that
isn't done here.

Backpatch-through: 14
Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/1407185.1766682319%40sss.pgh.pa.us

src/backend/jit/llvm/Makefile

@@ -22,12 +22,6 @@ endif
 PGFILEDESC = "llvmjit - JIT using LLVM"
 NAME = llvmjit
 
-# LLVM 14 produces deprecation warnings.  We'll need to make some changes
-# before the relevant functions are removed, but for now silence the warnings.
-ifeq ($(GCC), yes)
-LLVM_CFLAGS += -Wno-deprecated-declarations
-endif
-
 # All files in this directory use LLVM.
 CFLAGS += $(LLVM_CFLAGS)
 CXXFLAGS += $(LLVM_CXXFLAGS)