Remove debug-only private fields in release builds

We have a bunch of classes with private fields that are used only in
debug builds, some of them are probably opportunities for complete
removal. This is exposed by an upcoming commit that enforces the use of
"=delete" and "=default" throughout the codebase.

I've attempted to solve this by adding the GPOS_ASSERTS_ONLY attribute,
but GCC doesn't like that, throwing errors like the following:

In file included from ../src/backend/gporca/libgpos/include/gpos/common/clibwrapper.h:22,
                 from ../src/backend/gporca/libgpos/include/gpos/error/CMessage.h:21,
                 from ../src/backend/gporca/libgpos/include/gpos/error/CMessageTable.h:14,
                 from ../src/backend/gporca/libgpos/include/gpos/error/CMessageRepository.h:14,
                 from ../src/backend/gporca/libgpos/src/_api.cpp:15:
../src/backend/gporca/libgpos/include/gpos/attributes.h:15:43: error: ‘unused’ attribute ignored [-Werror=attributes]
   15 | #define GPOS_UNUSED __attribute__((unused))
      |                                           ^
../src/backend/gporca/libgpos/include/gpos/attributes.h:21:27: note: in expansion of macro ‘GPOS_UNUSED’
   21 | #define GPOS_ASSERTS_ONLY GPOS_UNUSED
      |                           ^~~~~~~~~~~
../src/backend/gporca/libgpos/include/gpos/memory/CAutoMemoryPool.h:56:31: note: in expansion of macro ‘GPOS_ASSERTS_ONLY’
   56 |  ELeakCheck m_leak_check_type GPOS_ASSERTS_ONLY;
      |                               ^~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors

So we're back to the good ol' #ifdef GPOS_DEBUG.

This is the first of a pair of manual changes. In the immediately
following commit I'll remove the remaining (unconditionally) unused
private fields.

src/backend/gporca/libgpopt/include/gpopt/base/CQueryContext.h

@@ -50,8 +50,10 @@ using namespace gpos;
 class CQueryContext
 {
 private:
+#ifdef GPOS_DEBUG
 	// memory pool
 	CMemoryPool *m_mp;
+#endif
 
 	// required plan properties in optimizer's produced plan
 	CReqdPropPlan *m_prpp;

src/backend/gporca/libgpopt/include/gpopt/search/CGroupExpression.h

@@ -71,8 +71,10 @@ public:
 		ShtCC;
 
 private:
+#ifdef GPOS_DEBUG
 	// memory pool
 	CMemoryPool *m_mp;
+#endif
 
 	// definition of context hash table accessor
 	typedef CSyncHashtableAccessByKey<CCostContext,	 // entry
@@ -182,7 +184,10 @@ private:
 
 	//private dummy ctor; used for creating invalid gexpr
 	CGroupExpression()
-		: m_mp(NULL),
+		:
+#ifdef GPOS_DEBUG
+		  m_mp(NULL),
+#endif
 		  m_id(GPOPT_INVALID_GEXPR_ID),
 		  m_pop(NULL),
 		  m_pdrgpgroup(NULL),

src/backend/gporca/libgpopt/src/base/CQueryContext.cpp

@@ -33,7 +33,10 @@ using namespace gpopt;
 CQueryContext::CQueryContext(CMemoryPool *mp, CExpression *pexpr,
 							 CReqdPropPlan *prpp, CColRefArray *colref_array,
 							 CMDNameArray *pdrgpmdname, BOOL fDeriveStats)
-	: m_mp(mp),
+	:
+#ifdef GPOS_DEBUG
+	  m_mp(mp),
+#endif
 	  m_prpp(prpp),
 	  m_pdrgpcr(colref_array),
 	  m_pdrgpcrSystemCols(NULL),

src/backend/gporca/libgpopt/src/search/CGroupExpression.cpp

@@ -50,7 +50,10 @@ CGroupExpression::CGroupExpression(CMemoryPool *mp, COperator *pop,
 								   CXform::EXformId exfid,
 								   CGroupExpression *pgexprOrigin,
 								   BOOL fIntermediate)
-	: m_mp(mp),
+	:
+#ifdef GPOS_DEBUG
+	  m_mp(mp),
+#endif
 	  m_id(GPOPT_INVALID_GEXPR_ID),
 	  m_pgexprDuplicate(NULL),
 	  m_pop(pop),

src/backend/gporca/libgpos/include/gpos/memory/CAutoMemoryPool.h

@@ -55,8 +55,10 @@ private:
 	// memory pool to protect
 	CMemoryPool *m_mp;
 
+#ifdef GPOS_DEBUG
 	// type of leak check to perform
 	ELeakCheck m_leak_check_type;
+#endif
 
 public:
 	// ctor

src/backend/gporca/libgpos/src/memory/CAutoMemoryPool.cpp

@@ -38,8 +38,10 @@ using namespace gpos;
 //  	the CMemoryPoolManager global instance
 //
 //---------------------------------------------------------------------------
-CAutoMemoryPool::CAutoMemoryPool(ELeakCheck leak_check_type)
+CAutoMemoryPool::CAutoMemoryPool(ELeakCheck leak_check_type GPOS_ASSERTS_ONLY)
+#ifdef GPOS_DEBUG
 	: m_leak_check_type(leak_check_type)
+#endif
 {
 	m_mp = CMemoryPoolManager::GetMemoryPoolMgr()->CreateMemoryPool();
 }

src/backend/gporca/libnaucrates/include/naucrates/md/CMDTypeGenericGPDB.h

@@ -71,8 +71,10 @@ private:
 	// is this a fixed-length type
 	BOOL m_is_fixed_length;
 
+#ifdef GPOS_DEBUG
 	// type length in number of bytes for fixed-length types, 0 otherwise
 	ULONG m_length;
+#endif
 
 	// is type passed by value or by reference
 	BOOL m_is_passed_by_value;

src/backend/gporca/libnaucrates/src/md/CMDTypeGenericGPDB.cpp

@@ -48,21 +48,23 @@ using namespace gpmd;
 //---------------------------------------------------------------------------
 CMDTypeGenericGPDB::CMDTypeGenericGPDB(
 	CMemoryPool *mp, IMDId *mdid, CMDName *mdname, BOOL is_redistributable,
-	BOOL is_fixed_length, ULONG length, BOOL is_passed_by_value,
-	IMDId *mdid_distr_opfamily, IMDId *mdid_legacy_distr_opfamily,
-	IMDId *mdid_op_eq, IMDId *mdid_op_neq, IMDId *mdid_op_lt,
-	IMDId *mdid_op_leq, IMDId *mdid_op_gt, IMDId *mdid_op_geq,
-	IMDId *mdid_op_cmp, IMDId *mdid_op_min, IMDId *mdid_op_max,
-	IMDId *mdid_op_avg, IMDId *mdid_op_sum, IMDId *mdid_op_count,
-	BOOL is_hashable, BOOL is_merge_joinable, BOOL is_composite_type,
-	BOOL is_text_related, IMDId *mdid_base_relation, IMDId *mdid_type_array,
-	INT gpdb_length)
+	BOOL is_fixed_length, ULONG length GPOS_ASSERTS_ONLY,
+	BOOL is_passed_by_value, IMDId *mdid_distr_opfamily,
+	IMDId *mdid_legacy_distr_opfamily, IMDId *mdid_op_eq, IMDId *mdid_op_neq,
+	IMDId *mdid_op_lt, IMDId *mdid_op_leq, IMDId *mdid_op_gt,
+	IMDId *mdid_op_geq, IMDId *mdid_op_cmp, IMDId *mdid_op_min,
+	IMDId *mdid_op_max, IMDId *mdid_op_avg, IMDId *mdid_op_sum,
+	IMDId *mdid_op_count, BOOL is_hashable, BOOL is_merge_joinable,
+	BOOL is_composite_type, BOOL is_text_related, IMDId *mdid_base_relation,
+	IMDId *mdid_type_array, INT gpdb_length)
 	: m_mp(mp),
 	  m_mdid(mdid),
 	  m_mdname(mdname),
 	  m_is_redistributable(is_redistributable),
 	  m_is_fixed_length(is_fixed_length),
+#ifdef GPOS_DEBUG
 	  m_length(length),
+#endif
 	  m_is_passed_by_value(is_passed_by_value),
 	  m_distr_opfamily(mdid_distr_opfamily),
 	  m_legacy_distr_opfamily(mdid_legacy_distr_opfamily),