Generate AST when importing a cpp file (#4790)

Ignore the AST and support a single Cpp import, for now.
Report cpp compilation errors and warnings.
Part of #4666

toolchain/check/import_cpp.cpp

@@ -0,0 +1,57 @@
+// 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/check/import_cpp.h"
+
+#include <memory>
+#include <string>
+
+#include "clang/Frontend/TextDiagnosticPrinter.h"
+#include "clang/Tooling/Tooling.h"
+#include "llvm/ADT/IntrusiveRefCntPtr.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/raw_ostream.h"
+#include "toolchain/check/context.h"
+#include "toolchain/check/diagnostic_helpers.h"
+#include "toolchain/diagnostics/diagnostic.h"
+#include "toolchain/diagnostics/format_providers.h"
+
+namespace Carbon::Check {
+
+auto ImportCppFile(Context& context, SemIRLoc loc, llvm::StringRef file_path,
+                   llvm::StringRef code) -> void {
+  std::string diagnostics_str;
+  llvm::raw_string_ostream diagnostics_stream(diagnostics_str);
+
+  llvm::IntrusiveRefCntPtr<clang::DiagnosticOptions> diagnostic_options(
+      new clang::DiagnosticOptions());
+  clang::TextDiagnosticPrinter diagnostics_consumer(diagnostics_stream,
+                                                    diagnostic_options.get());
+  // TODO: Share compilation flags with ClangRunner.
+  auto ast = clang::tooling::buildASTFromCodeWithArgs(
+      code, {}, file_path, "clang-tool",
+      std::make_shared<clang::PCHContainerOperations>(),
+      clang::tooling::getClangStripDependencyFileAdjuster(),
+      clang::tooling::FileContentMappings(), &diagnostics_consumer);
+  // TODO: Implement and use a DynamicRecursiveASTVisitor to traverse the AST.
+  int num_errors = diagnostics_consumer.getNumErrors();
+  int num_warnings = diagnostics_consumer.getNumWarnings();
+  if (num_errors > 0) {
+    // TODO: Remove the warnings part when there are no warnings.
+    CARBON_DIAGNOSTIC(
+        CppInteropParseError, Error,
+        "{0} error{0:s} and {1} warning{1:s} in `Cpp` import `{2}`:\n{3}",
+        IntAsSelect, IntAsSelect, std::string, std::string);
+    context.emitter().Emit(loc, CppInteropParseError, num_errors, num_warnings,
+                           file_path.str(), diagnostics_str);
+  } else if (num_warnings > 0) {
+    CARBON_DIAGNOSTIC(CppInteropParseWarning, Warning,
+                      "{0} warning{0:s} in `Cpp` import `{1}`:\n{2}",
+                      IntAsSelect, std::string, std::string);
+    context.emitter().Emit(loc, CppInteropParseWarning, num_warnings,
+                           file_path.str(), diagnostics_str);
+  }
+}
+
+}  // namespace Carbon::Check