Rename parser dir to parse (#3178)

Continuing with #3070. Just a dir and file rename (mostly removing
prefixes, although for parse_tree_fuzzer and parse_tree_file_test I'm
dropping "tree" instead of "parse"). Everything in the parse dir should
be marked as a move.

toolchain/parse/handle_declaration_scope_loop.cpp

@@ -0,0 +1,67 @@
+// 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
+
+#include "toolchain/parse/context.h"
+
+namespace Carbon::Parse {
+
+// Handles an unrecognized declaration, adding an error node.
+static auto HandleUnrecognizedDeclaration(Context& context) -> void {
+  CARBON_DIAGNOSTIC(UnrecognizedDeclaration, Error,
+                    "Unrecognized declaration introducer.");
+  context.emitter().Emit(*context.position(), UnrecognizedDeclaration);
+  auto cursor = *context.position();
+  auto semi = context.SkipPastLikelyEnd(cursor);
+  // Locate the EmptyDeclaration at the semi when found, but use the
+  // original cursor location for an error when not.
+  context.AddLeafNode(NodeKind::EmptyDeclaration, semi ? *semi : cursor,
+                      /*has_error=*/true);
+}
+
+auto HandleDeclarationScopeLoop(Context& context) -> void {
+  // This maintains the current state unless we're at the end of the scope.
+
+  switch (context.PositionKind()) {
+    case Lex::TokenKind::CloseCurlyBrace:
+    case Lex::TokenKind::EndOfFile: {
+      // This is the end of the scope, so the loop state ends.
+      context.PopAndDiscardState();
+      break;
+    }
+    case Lex::TokenKind::Class: {
+      context.PushState(State::TypeIntroducerAsClass);
+      break;
+    }
+    case Lex::TokenKind::Constraint: {
+      context.PushState(State::TypeIntroducerAsNamedConstraint);
+      break;
+    }
+    case Lex::TokenKind::Fn: {
+      context.PushState(State::FunctionIntroducer);
+      break;
+    }
+    case Lex::TokenKind::Interface: {
+      context.PushState(State::TypeIntroducerAsInterface);
+      break;
+    }
+    case Lex::TokenKind::Namespace: {
+      context.PushState(State::Namespace);
+      break;
+    }
+    case Lex::TokenKind::Semi: {
+      context.AddLeafNode(NodeKind::EmptyDeclaration, context.Consume());
+      break;
+    }
+    case Lex::TokenKind::Var: {
+      context.PushState(State::VarAsSemicolon);
+      break;
+    }
+    default: {
+      HandleUnrecognizedDeclaration(context);
+      break;
+    }
+  }
+}
+
+}  // namespace Carbon::Parse