Make plpgsql use its DTYPE_REC code paths for composite-type variables.

Formerly, DTYPE_REC was used only for variables declared as "record";
variables of named composite types used DTYPE_ROW, which is faster for
some purposes but much less flexible.  In particular, the ROW code paths
are entirely incapable of dealing with DDL-caused changes to the number
or data types of the columns of a row variable, once a particular plpgsql
function has been parsed for the first time in a session.  And, since the
stored representation of a ROW isn't a tuple, there wasn't any easy way
to deal with variables of domain-over-composite types, since the domain
constraint checking code would expect the value to be checked to be a
tuple.  A lesser, but still real, annoyance is that ROW format cannot
represent a true NULL composite value, only a row of per-field NULL
values, which is not exactly the same thing.

Hence, switch to using DTYPE_REC for all composite-typed variables,
whether "record", named composite type, or domain over named composite
type.  DTYPE_ROW remains but is used only for its native purpose, to
represent a fixed-at-compile-time list of variables, for instance the
targets of an INTO clause.

To accomplish this without taking significant performance losses, introduce
infrastructure that allows storing composite-type variables as "expanded
objects", similar to the "expanded array" infrastructure introduced in
commit 1dc5ebc90.  A composite variable's value is thereby kept (most of
the time) in the form of separate Datums, so that field accesses and
updates are not much more expensive than they were in the ROW format.
This holds the line, more or less, on performance of variables of named
composite types in field-access-intensive microbenchmarks, and makes
variables declared "record" perform much better than before in similar
tests.  In addition, the logic involved with enforcing composite-domain
constraints against updates of individual fields is in the expanded
record infrastructure not plpgsql proper, so that it might be reusable
for other purposes.

In further support of this, introduce a typcache feature for assigning a
unique-within-process identifier to each distinct tuple descriptor of
interest; in particular, DDL alterations on composite types result in a new
identifier for that type.  This allows very cheap detection of the need to
refresh tupdesc-dependent data.  This improves on the "tupDescSeqNo" idea
I had in commit 687f096ea: that assigned identifying sequence numbers to
successive versions of individual composite types, but the numbers were not
unique across different types, nor was there support for assigning numbers
to registered record types.

In passing, allow plpgsql functions to accept as well as return type
"record".  There was no good reason for the old restriction, and it
was out of step with most of the other PLs.

Tom Lane, reviewed by Pavel Stehule

Discussion: https://postgr.es/m/8962.1514399547@sss.pgh.pa.us

src/include/utils/typcache.h

@@ -76,11 +76,14 @@ typedef struct TypeCacheEntry
 	/*
 	 * Tuple descriptor if it's a composite type (row type).  NULL if not
 	 * composite or information hasn't yet been requested.  (NOTE: this is a
-	 * reference-counted tupledesc.)  To simplify caching dependent info,
-	 * tupDescSeqNo is incremented each time tupDesc is rebuilt in a session.
+	 * reference-counted tupledesc.)
+	 *
+	 * To simplify caching dependent info, tupDesc_identifier is an identifier
+	 * for this tupledesc that is unique for the life of the process, and
+	 * changes anytime the tupledesc does.  Zero if not yet determined.
 	 */
 	TupleDesc	tupDesc;
-	int64		tupDescSeqNo;
+	uint64		tupDesc_identifier;
 
 	/*
 	 * Fields computed when TYPECACHE_RANGE_INFO is requested.  Zeroes if not
@@ -138,6 +141,9 @@ typedef struct TypeCacheEntry
 #define TYPECACHE_HASH_EXTENDED_PROC		0x4000
 #define TYPECACHE_HASH_EXTENDED_PROC_FINFO	0x8000
 
+/* This value will not equal any valid tupledesc identifier, nor 0 */
+#define INVALID_TUPLEDESC_IDENTIFIER ((uint64) 1)
+
 /*
  * Callers wishing to maintain a long-lived reference to a domain's constraint
  * set must store it in one of these.  Use InitDomainConstraintRef() and
@@ -179,6 +185,8 @@ extern TupleDesc lookup_rowtype_tupdesc_domain(Oid type_id, int32 typmod,
 
 extern void assign_record_type_typmod(TupleDesc tupDesc);
 
+extern uint64 assign_record_type_identifier(Oid type_id, int32 typmod);
+
 extern int	compare_values_of_enum(TypeCacheEntry *tcache, Oid arg1, Oid arg2);
 
 extern size_t SharedRecordTypmodRegistryEstimate(void);