Pass collations to functions in FunctionCallInfoData, not FmgrInfo.

Since collation is effectively an argument, not a property of the function,
FmgrInfo is really the wrong place for it; and this becomes critical in
cases where a cached FmgrInfo is used for varying purposes that might need
different collation settings.  Fix by passing it in FunctionCallInfoData
instead.  In particular this allows a clean fix for bug #5970 (record_cmp
not working).  This requires touching a bit more code than the original
method, but nobody ever thought that collations would not be an invasive
patch...

src/include/access/gin_private.h

@@ -303,6 +303,8 @@ typedef struct GinState
 	FmgrInfo	comparePartialFn[INDEX_MAX_KEYS];		/* optional method */
 	/* canPartialMatch[i] is true if comparePartialFn[i] is valid */
 	bool		canPartialMatch[INDEX_MAX_KEYS];
+	/* Collations to supply to the compareFns and comparePartialFns */
+	Oid			compareCollation[INDEX_MAX_KEYS];
 } GinState;
 
 /* XLog stuff */