Revise collation derivation method and expression-tree representation.

All expression nodes now have an explicit output-collation field, unless
they are known to only return a noncollatable data type (such as boolean
or record).  Also, nodes that can invoke collation-aware functions store
a separate field that is the collation value to pass to the function.
This avoids confusion that arises when a function has collatable inputs
and noncollatable output type, or vice versa.

Also, replace the parser's on-the-fly collation assignment method with
a post-pass over the completed expression tree.  This allows us to use
a more complex (and hopefully more nearly spec-compliant) assignment
rule without paying for it in extra storage in every expression node.

Fix assorted bugs in the planner's handling of collations by making
collation one of the defining properties of an EquivalenceClass and
by converting CollateExprs into discardable RelabelType nodes during
expression preprocessing.

src/include/fmgr.h

@@ -40,6 +40,11 @@ typedef Datum (*PGFunction) (FunctionCallInfo fcinfo);
  * before a function can be called through fmgr.  If the same function is
  * to be called multiple times, the lookup need be done only once and the
  * info struct saved for re-use.
+ *
+ * Note that fn_collation and fn_expr really are parse-time-determined
+ * information about the arguments, rather than about the function itself.
+ * But it's convenient to store them here rather than in FunctionCallInfoData,
+ * where they might more logically belong.
  */
 typedef struct FmgrInfo
 {
@@ -50,7 +55,7 @@ typedef struct FmgrInfo
 	bool		fn_strict;		/* function is "strict" (NULL in => NULL out) */
 	bool		fn_retset;		/* function returns a set */
 	unsigned char fn_stats;		/* collect stats if track_functions > this */
-	Oid			fn_collation;	/* collation to use */
+	Oid			fn_collation;	/* collation that function should use */
 	void	   *fn_extra;		/* extra space for use by handler */
 	MemoryContext fn_mcxt;		/* memory context to store fn_extra in */
 	fmNodePtr	fn_expr;		/* expression parse tree for call, or NULL */
@@ -84,15 +89,11 @@ extern void fmgr_info(Oid functionId, FmgrInfo *finfo);
 extern void fmgr_info_cxt(Oid functionId, FmgrInfo *finfo,
 			  MemoryContext mcxt);
 
-/*
- * Initialize the fn_collation field
- */
-extern void fmgr_info_collation(Oid collationId, FmgrInfo *finfo);
-
-/*
- * Initialize the fn_expr field and set the collation based on it
- */
-extern void fmgr_info_expr(fmNodePtr expr, FmgrInfo *finfo);
+/* Macros for setting the fn_collation and fn_expr fields */
+#define fmgr_info_set_collation(collationId, finfo) \
+	((finfo)->fn_collation = (collationId))
+#define fmgr_info_set_expr(expr, finfo) \
+	((finfo)->fn_expr = (expr))
 
 /*
  * Copy an FmgrInfo struct
@@ -146,6 +147,12 @@ extern void fmgr_info_copy(FmgrInfo *dstinfo, FmgrInfo *srcinfo,
 /* Standard parameter list for fmgr-compatible functions */
 #define PG_FUNCTION_ARGS	FunctionCallInfo fcinfo
 
+/*
+ * Get collation function should use.
+ */
+#define PG_GET_COLLATION() \
+	(fcinfo->flinfo ? fcinfo->flinfo->fn_collation : InvalidOid)
+
 /*
  * Get number of arguments passed to function.
  */
@@ -307,7 +314,6 @@ extern struct varlena *pg_detoast_datum_packed(struct varlena * datum);
 #define PG_RETURN_VARCHAR_P(x) PG_RETURN_POINTER(x)
 #define PG_RETURN_HEAPTUPLEHEADER(x)  PG_RETURN_POINTER(x)
 
-#define PG_GET_COLLATION()		(fcinfo->flinfo ? fcinfo->flinfo->fn_collation : InvalidOid)
 
 /*-------------------------------------------------------------------------
  *		Support for detecting call convention of dynamically-loaded functions
@@ -450,7 +456,7 @@ extern Datum DirectFunctionCall9(PGFunction func, Datum arg1, Datum arg2,
 					Datum arg6, Datum arg7, Datum arg8,
 					Datum arg9);
 
-/* the same but passing a collation */
+/* The same, but passing a collation to use */
 extern Datum DirectFunctionCall1WithCollation(PGFunction func, Oid collation,
 											  Datum arg1);
 extern Datum DirectFunctionCall2WithCollation(PGFunction func, Oid collation,