Split Parse out to its own target. (#3556)

This is mirroring the structure of codegen/codegen.h, lower/lower.h, and
check/check.h. I recently did lex/lex.h, so parse/parse.h is the last.
Now, the directory's main API file is eponymous with the directory.

I could've used a friend function to avoid making the Tree constructor
public, but in other places we make less use of `friend`, just leaving
things public. This felt more consistent, and simple because it only
affects the constructor.

toolchain/parse/parse.cpp

@@ -0,0 +1,61 @@
+// 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 "common/check.h"
+#include "toolchain/base/pretty_stack_trace_function.h"
+#include "toolchain/parse/context.h"
+#include "toolchain/parse/node_kind.h"
+#include "toolchain/parse/typed_nodes.h"
+
+namespace Carbon::Parse {
+
+auto HandleInvalid(Context& context) -> void {
+  CARBON_FATAL() << "The Invalid state shouldn't be on the stack: "
+                 << context.PopState();
+}
+
+auto Parse(Lex::TokenizedBuffer& tokens, DiagnosticConsumer& consumer,
+           llvm::raw_ostream* vlog_stream) -> Tree {
+  Lex::TokenLocationTranslator translator(&tokens);
+  Lex::TokenDiagnosticEmitter emitter(translator, consumer);
+
+  // Delegate to the parser.
+  Tree tree(tokens);
+  Context context(tree, tokens, emitter, vlog_stream);
+  PrettyStackTraceFunction context_dumper(
+      [&](llvm::raw_ostream& output) { context.PrintForStackDump(output); });
+
+  context.AddLeafNode(NodeKind::FileStart,
+                      context.ConsumeChecked(Lex::TokenKind::FileStart));
+
+  context.PushState(State::DeclScopeLoop);
+
+  while (!context.state_stack().empty()) {
+    // clang warns on unhandled enum values; clang-tidy is incorrect here.
+    // NOLINTNEXTLINE(bugprone-switch-missing-default-case)
+    switch (context.state_stack().back().state) {
+#define CARBON_PARSE_STATE(Name) \
+  case State::Name:              \
+    Handle##Name(context);       \
+    break;
+#include "toolchain/parse/state.def"
+    }
+  }
+
+  context.AddLeafNode(NodeKind::FileEnd, *context.position());
+
+  if (auto verify = tree.Verify(); !verify.ok()) {
+    // TODO: This is temporarily printing to stderr directly during development.
+    // If we can, restrict this to a subtree with the error and add it to the
+    // stack trace (such as with PrettyStackTraceFunction). Otherwise, switch
+    // back to vlog_stream prior to broader distribution so that end users are
+    // hopefully comfortable copy-pasting stderr when there are bugs in tree
+    // construction.
+    tree.Print(llvm::errs());
+    CARBON_FATAL() << "Invalid tree returned by Parse(): " << verify.error();
+  }
+  return tree;
+}
+
+}  // namespace Carbon::Parse