Use a filename without a line number as a cue for autoupdate. (#4677)

This is so that diagnostics which lack a location get split file
clustering.

testing/file_test/file_test_base.cpp

@@ -277,7 +277,7 @@ auto FileTestBase::GetLineNumberReplacements(
     -> llvm::SmallVector<LineNumberReplacement> {
   return {{.has_file = true,
            .re = std::make_shared<RE2>(
-               llvm::formatv(R"(({0}):(\d+))", llvm::join(filenames, "|"))),
+               llvm::formatv(R"(({0}):(\d+)?)", llvm::join(filenames, "|"))),
            .line_formatv = R"({0})"}};
 }