Add a debugging option to stress-test outfuncs.c and readfuncs.c.

In the normal course of operation, query trees will be serialized only if
they are stored as views or rules; and plan trees will be serialized only
if they get passed to parallel-query workers.  This leaves an awful lot of
opportunity for bugs/oversights to not get detected, as indeed we've just
been reminded of the hard way.

To improve matters, this patch adds a new compile option
WRITE_READ_PARSE_PLAN_TREES, which is modeled on the longstanding option
COPY_PARSE_PLAN_TREES; but instead of passing all parse and plan trees
through copyObject, it passes them through nodeToString + stringToNode.
Enabling this option in a buildfarm animal or two will catch problems
at least for cases that are exercised by the regression tests.

A small problem with this idea is that readfuncs.c historically has
discarded location fields, on the reasonable grounds that parse
locations in a retrieved view are not relevant to the current query.
But doing that in WRITE_READ_PARSE_PLAN_TREES breaks pg_stat_statements,
and it could cause problems for future improvements that might try to
report error locations at runtime.  To fix that, provide a variant
behavior in readfuncs.c that makes it restore location fields when
told to.

In passing, const-ify the string arguments of stringToNode and its
subsidiary functions, just because it annoyed me that they weren't
const already.

Discussion: https://postgr.es/m/17114.1537138992@sss.pgh.pa.us

src/include/nodes/nodes.h

@@ -610,7 +610,10 @@ extern char *bmsToString(const struct Bitmapset *bms);
 /*
  * nodes/{readfuncs.c,read.c}
  */
-extern void *stringToNode(char *str);
+extern void *stringToNode(const char *str);
+#ifdef WRITE_READ_PARSE_PLAN_TREES
+extern void *stringToNodeWithLocations(const char *str);
+#endif
 extern struct Bitmapset *readBitmapset(void);
 extern uintptr_t readDatum(bool typbyval);
 extern bool *readBoolCols(int numCols);