Refactor file_test autoupdating into a class. (#3228)

I'm finding the current autoupdate difficult to reason about. What I'm
trying to do here is use the class to make it easier to add helper
functions.

For example, I merge the vector+cursor for stdout/stderr into an object,
passed to helpers together instead of as two parameters.
ShouldAddCheckLine can check against output_file_number_ without passing
that through a couple levels of function calls. In turn,
ShouldAddCheckLine is shared with the end-of-file logic instead of that
having its own comparison from what AddCheckLines does.

Also, I'm trying to get the pre-AUTOUPDATE edits in their own loop,
distinct from the main code. The class means AddRemappedNonCheckLine is
a helper function to share code, instead of a lambda (which I was
thinking would just confuse the flow further).

I'm also changing the input non_check_lines to a single vector to match
stdout/stderr. Because of the AUTOUPDATE + SPLIT lines, we're guaranteed
to have at least one line per file.

I realized file_offset_in_new_lines is redundant with output_line_number
so code now uses the latter (because it's older -- I think one's as good
as the other, otherwise).

Note, this change deliberately does not affect output. I'm only trying
to make it easier to read for the next changes, using the lack of change
in results as a good indicator that this is getting it right.

testing/file_test/file_test_base.cpp

@@ -19,6 +19,7 @@
 #include "llvm/Support/InitLLVM.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/PrettyStackTrace.h"
+#include "testing/file_test/autoupdate.h"
 
 ABSL_FLAG(std::vector<std::string>, file_tests, {},
           "A comma-separated list of repo-relative names of test files. "
@@ -111,7 +112,7 @@ auto FileTestBase::Autoupdate() -> ErrorOr<bool> {
 
   llvm::SmallVector<llvm::StringRef> filenames;
   filenames.reserve(context.non_check_lines.size());
-  if (context.non_check_lines.size() > 1) {
+  if (context.has_splits) {
     // There are splits, so we provide an empty name for the first file.
     filenames.push_back({});
   }
@@ -124,12 +125,14 @@ auto FileTestBase::Autoupdate() -> ErrorOr<bool> {
     expected_filenames = expected_filenames.drop_front();
   }
 
-  return AutoupdateFileTest(
-      std::filesystem::absolute(test_name_.str()), context.input_content,
-      filenames, *context.autoupdate_line_number, context.non_check_lines,
-      context.stdout, context.stderr, GetDefaultFileRE(expected_filenames),
-      GetLineNumberReplacements(expected_filenames),
-      [&](std::string& line) { DoExtraCheckReplacements(line); });
+  return FileTestAutoupdater(
+             std::filesystem::absolute(test_name_.str()), context.input_content,
+             filenames, *context.autoupdate_line_number,
+             context.non_check_lines, context.stdout, context.stderr,
+             GetDefaultFileRE(expected_filenames),
+             GetLineNumberReplacements(expected_filenames),
+             [&](std::string& line) { DoExtraCheckReplacements(line); })
+      .Run();
 }
 
 auto FileTestBase::GetLineNumberReplacements(
@@ -253,7 +256,7 @@ auto FileTestBase::ProcessTestFile(TestContext& context) -> ErrorOr<Success> {
   // The current file's start.
   const char* current_file_start = nullptr;
 
-  context.non_check_lines.resize(1);
+  int file_number = 0;
   while (!cursor.empty()) {
     auto [line, next_cursor] = cursor.split("\n");
     cursor = next_cursor;
@@ -269,7 +272,9 @@ auto FileTestBase::ProcessTestFile(TestContext& context) -> ErrorOr<Success> {
                << "AUTOUPDATE/NOAUTOUPDATE setting must be in the first file.";
       }
 
-      context.non_check_lines.push_back({FileTestLine(0, line)});
+      context.has_splits = true;
+      ++file_number;
+      context.non_check_lines.push_back(FileTestLine(file_number, 0, line));
       // On a file split, add the previous file, then start a new one.
       if (current_file_start) {
         context.test_files.push_back(TestFile(
@@ -311,7 +316,8 @@ auto FileTestBase::ProcessTestFile(TestContext& context) -> ErrorOr<Success> {
         expected->push_back(check_matcher);
       }
     } else {
-      context.non_check_lines.back().push_back(FileTestLine(line_index, line));
+      context.non_check_lines.push_back(
+          FileTestLine(file_number, line_index, line));
       if (line_trimmed.consume_front("// ARGS: ")) {
         if (context.test_args.empty()) {
           // Split the line into arguments.