Extend deduced and regular parameter handling to types. (#2684)

This makes it possible to specify both deduced and regular parameters on types. It reorganizes the handling of parameter lists in order to allow more reuse of code in this approach. Both functions and types use the new DeclarationNameAndParams handling. Overall the goal here is to take advantage of commonality in structure.

Regarding destructors, the likely approach would be to use ParameterListAsDeduced directly because `destructor` is a keyword with no declaration name and no regular parameters.

toolchain/parser/testdata/function/declaration/deduced_empty.carbon

@@ -0,0 +1,18 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{carbon-run-parser}
+// CHECK:STDOUT: [
+// CHECK:STDOUT:   {kind: 'FunctionIntroducer', text: 'fn'},
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'foo'},
+// CHECK:STDOUT:     {kind: 'DeducedParameterListStart', text: '['},
+// CHECK:STDOUT:   {kind: 'DeducedParameterList', text: ']', subtree_size: 2},
+// CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 2},
+// CHECK:STDOUT: {kind: 'FunctionDeclaration', text: ';', subtree_size: 7},
+// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT: ]
+
+fn foo[]();