Fold deduced pattern parsing into the general pattern parsing. (#2649)

Depends on #2646 

Right now, deduced parameter handling is very narrow to `self` support. This folds it into pattern handling, which should eventually be a superset of deduced parameter support, so this will avoid more duplication of logic.

Note, this subtly adds handling of multiple deduced parameters, but not generic parameters (`:!`) so it's still not quite right.

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

@@ -0,0 +1,21 @@
+// 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: 'ParameterListStart', text: '('},
+// CHECK:STDOUT:         {kind: 'DeclaredName', text: 'a'},
+// CHECK:STDOUT:           {kind: 'Literal', text: 'i32'},
+// CHECK:STDOUT:         {kind: 'PostfixOperator', text: '*', subtree_size: 2},
+// CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 4},
+// CHECK:STDOUT:     {kind: 'Address', text: 'addr', subtree_size: 5},
+// CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 7},
+// CHECK:STDOUT: {kind: 'FunctionDeclaration', text: ';', subtree_size: 10},
+// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT: ]
+
+fn foo(addr a: i32*);