Unify DeclaredName and DesignatedName as just Identifier (#2939)

This is just a simplification: I think these different forms are getting in the way more than they're helping, particularly as I was looking into namespace functionality. The handling in semantics can be identical, providing a more uniform behavior.

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

@@ -5,7 +5,7 @@
 // AUTOUPDATE
 // CHECK:STDOUT: [
 // CHECK:STDOUT:   {kind: 'FunctionIntroducer', text: 'fn'},
-// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'F'},
+// CHECK:STDOUT:   {kind: 'Identifier', text: 'F'},
 // CHECK:STDOUT:     {kind: 'ParameterListStart', text: '('},
 // CHECK:STDOUT:   {kind: 'ParameterList', text: ')', subtree_size: 2},
 // CHECK:STDOUT: {kind: 'FunctionDeclaration', text: ';', subtree_size: 5},