Track the location of the Cpp import for use in Clang diagnostics. (#5783)

When diagnosing a problem with C++ code imported from Carbon, include
the location of the specific `import Cpp` statement that imported the
C++ code as part of the backtrace, rather than providing a location in a
generated file that doesn't exist on disk.

Also fix handling in autoupdate of check lines that contain multiple
file name and line number pairs to use the matched file name for
remapping of locations rather than the first file name in the line.

toolchain/check/import_cpp.cpp

@@ -13,6 +13,7 @@
 #include "clang/Frontend/TextDiagnostic.h"
 #include "clang/Sema/Lookup.h"
 #include "clang/Tooling/Tooling.h"
+#include "common/ostream.h"
 #include "common/raw_string_ostream.h"
 #include "llvm/ADT/IntrusiveRefCntPtr.h"
 #include "llvm/ADT/StringRef.h"
@@ -44,6 +45,15 @@ static auto GenerateCppIncludesHeaderCode(
   std::string code;
   llvm::raw_string_ostream code_stream(code);
   for (const Parse::Tree::PackagingNames& import : imports) {
+    // Add a line marker directive pointing at the location of the `import Cpp`
+    // declaration in the Carbon source file. This will cause Clang's
+    // diagnostics machinery to track and report the location in Carbon code
+    // where the import was written.
+    auto token = context.parse_tree().node_token(import.node_id);
+    code_stream << "# " << context.tokens().GetLineNumber(token) << " \""
+                << FormatEscaped(context.tokens().source().filename())
+                << "\"\n";
+
     code_stream << "#include \""
                 << FormatEscaped(
                        context.string_literal_values().Get(import.library_id))
@@ -108,6 +118,7 @@ class CarbonClangDiagnosticConsumer : public clang::DiagnosticConsumer {
     // TODO: Consider allowing setting `DiagnosticOptions` or use
     // `ASTUnit::getDiagnostics().getLangOptions().getDiagnosticOptions()`.
     clang::DiagnosticOptions diagnostic_options;
+    diagnostic_options.ShowPresumedLoc = true;
     clang::TextDiagnostic text_diagnostic(diagnostics_stream, lang_options,
                                           diagnostic_options);
     text_diagnostic.emitDiagnostic(
@@ -139,8 +150,6 @@ class CarbonClangDiagnosticConsumer : public clang::DiagnosticConsumer {
         case clang::DiagnosticsEngine::Warning:
         case clang::DiagnosticsEngine::Error:
         case clang::DiagnosticsEngine::Fatal: {
-          // TODO: Parse the message to select the relevant C++ import and add
-          // that information to the location.
           CARBON_DIAGNOSTIC(CppInteropParseWarning, Warning, "{0}",
                             std::string);
           CARBON_DIAGNOSTIC(CppInteropParseError, Error, "{0}", std::string);