Delay parse analysis and rewrite until we're ready to execute the query.

This change fixes a longstanding bugaboo with SQL functions: you could
not write DDL that would affect later statements in the same function.
That's mostly still true with new-style SQL functions, since the
results of parse analysis are baked into the stored query trees (and
protected by dependency records).  But for old-style SQL functions,
it will now work much as it does with plpgsql functions.

The key changes required are to (1) stash the parsetrees read from
pg_proc somewhere safe until we're ready to process them, and (2)
adjust the error context reporting.  sql_compile_error_callback is now
only useful for giving context for errors detected by raw parsing.
Errors detected in either parse analysis or planning are handled by
sql_exec_error_callback, as they were before this patch series.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/8216639.NyiUUSuA9g@aivenlaptop

Author: tglsfdc

doc/src/sgml/xfunc.sgml

@@ -234,21 +234,6 @@ CALL clean_emp();
      whereas returning <type>void</type> is a PostgreSQL extension.
     </para>
 
-    <note>
-     <para>
-      The entire body of an SQL function is parsed before any of it is
-      executed.  While an SQL function can contain commands that alter
-      the system catalogs (e.g., <command>CREATE TABLE</command>), the effects
-      of such commands will not be visible during parse analysis of
-      later commands in the function.  Thus, for example,
-      <literal>CREATE TABLE foo (...); INSERT INTO foo VALUES(...);</literal>
-      will not work as desired if packaged up into a single SQL function,
-      since <structname>foo</structname> won't exist yet when the <command>INSERT</command>
-      command is parsed.  It's recommended to use <application>PL/pgSQL</application>
-      instead of an SQL function in this type of situation.
-     </para>
-   </note>
-
    <para>
     The syntax of the <command>CREATE FUNCTION</command> command requires
     the function body to be written as a string constant.  It is usually