[dev.typeparams] cmd/compile: add derived types and subdictionaries to dictionaries

This is code in progress to generate the two main other types of entries
in dictionaries:
 - all types in the instantiated function derived from the type
   arguments (which are currently concrete, but will eventually be
   gcshapes)
 - pointers (i.e. mainly the unique name) to all needed sub-dictionaries

In order to generate these entries, we now generate cached information
gfInfo about generic functions/methods that can be used for creating the
instantiated dictionaries. We use the type substituter to compute the
right type args for instantiated sub-dictionaries.

If infoPrintMode is changed to true, the code prints out all the
information gathered about generic functions, and also the entries in
all the dictionaries that are instantiated. The debug mode also prints
out the locations where we need main dictionaries in non-instantiated
functions.

Other changes:
 - Moved the dictionary generation back to stencil.go from reflect.go,
   since we need to do extra analysis for the new dictionary entries. In
   the process, made getInstantiation generate both the function
   instantiation and the associated dictionary.

 - Put in small change for now in reflect.go, so that we don't try
   generate separate dictionaries for Value[T].get and the
   auto-generated (*Value[T]).get.  The auto-generated wrapper shouldn't really
   need a dictionary.

 - Detected, but not handling yet, a new case which needs
   dictionaries - closures that have function params or captured
   variables whose types are derived from type arguments.

 - Added new tests in dictionaryCapture for use of method
   value/expressions in generic functions and for mutually recursive
   generic functions.

Change-Id: If0cbde8805a9f673a23f5ec798769c85c9c5359b
Reviewed-on: https://go-review.googlesource.com/c/go/+/327311
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ir/expr.go

@@ -700,6 +700,11 @@ func (n *UnaryExpr) SetOp(op Op) {
 	}
 }
 
+// Probably temporary: using Implicit() flag to mark generic function nodes that
+// are called to make getGfInfo analysis easier in one pre-order pass.
+func (n *InstExpr) Implicit() bool     { return n.flags&miniExprImplicit != 0 }
+func (n *InstExpr) SetImplicit(b bool) { n.flags.set(miniExprImplicit, b) }
+
 // An InstExpr is a generic function or type instantiation.
 type InstExpr struct {
 	miniExpr