This commit replaces two pre-C++ 11 practices with their modern, more
intent-expressive equivalents: "disallowed" copy and assignments, and
pairs of empty braces for special member function (destructors, default
constructors, and etc) bodies.

"Disallowed" copy constructors / assignment
-------------------------------------------
Old: private, unimplemented copy constructors and copy-assignment
operators in classes. These are usually paired with a semi-descriptive
comment.

New: publicly declare them as "= delete".

Fun fact: we had 13 spellings in comments on disallowed copy
constructors:

01. disable copy ctor
02. hidden copy ctor
03. inaccessible copy ctor
04. no copy ctor
05. no default copy ctor
06. private copy ctor
07. private no copy ctor
08. disabled copy constructor
09. no copy constructor
10. private copy constructor
11. copy c'tor - not defined
12. no copy c'tor
13. private copy c'tor

This commit removes all of them, because the new code ("~T() = delete")
already clearly expresses the intent of disallowing copy and assignment.

To keep the history clear, this commit leaves the declaration of
"prohibited" functions private. A forthcoming commit will wholesale
change them to public.

Defaulted special member functions
----------------------------------
Old:
struct A {
  A() {}
  ~A();
};
A::~A() {}

New:
struct A {
  A() = default;
  ~A();
};
A::~A() = default;

Replacing empty braces with defaulting not only makes them more clear,
they also enable more opportunities in compiler optimizations, as e.g.
some defaulted functions might be recognized as trivial.

Most of this commit is produced by running clang-tidy with an invocation
like the following (plus some CMake and shell tricks):

clang-tidy-12 -header-filter 'gpdbcost|gpopt|gpos|naucrates' -checks '-*,modernize-use-equals-delete,modernize-use-equals-default'

The tool uses a slightly conservative heuristic to detect a large
portion of the two outdated patterns above and rewrite them into using
"= delete" and "= default". Making the "= delete" functions public, is
sadly a FIXME item, so we'll have to do it by hand (in a forthcoming
commit).

The canonical config file is in src/backend/gporca/.clang-format, I've
created two symlinks, one for GPOPT headers, one for GPOPT.

* Make DbgPrint and OsPrint methods on CRefCount

Create a single DbgPrint() method on the CRefCount class. Also create
a virtual OsPrint() method, making some objects derived from CRefCount
easier to print from the debugger.

Note that not all the OsPrint methods had the same signatures, some
additional OsPrintxxx() methods have been generated for that.

* Making print output easier to read, print some stuff on demand

Required columns in required plan properties are always the same
for a given group. Also, equivalent expressions in required distribution
properties are important in certain cases, but in most cases they
disrupt the display and make it harder to read.

Added two traceflags, EopttracePrintRequiredColumns and
EopttracePrintEquivDistrSpecs that have to be set to print this
information. If you want to go back to the old display, use these
options when running gporca_test: -T 101016 -T 101017

* Add support for printing alternative plans

A new method, CEngine::DbgPrintExpr() can be called from
COptimizer::PexprOptimize, to allow printing of the best plan
for different contexts. This is only enabled in debug builds.

To use this:

- run an MDP using gporca_test, using a debug build
- print out memo after optimization (-T 101006 -T 101010)
- set a breakpoint near the end of COptimizer::PexprOptimize()
- if, after looking at the contents of memo, you want to see
  the optimal plan for context c of group g, do the following:
  p eng.DbgPrintExpr(g, c)

You could also get the same info from the memo printout, but it
would take a lot longer.