Modernize code using c++17 constructs

Mostly done using

find . \( -name '*.cc' -o -name '*.h' \) -a -type f -exec clang-tidy -p \
cmake-build -checks='-*,google-*,modernize-*,-modernize-use-nodiscard,-modernize-use-trailing-return-type' {} -fix \;

Change-Id: Ifccbcabe7a1d9a32a09d28ac4f3f8466696c1a50
diff --git a/internal/ceres/dense_cholesky_test.cc b/internal/ceres/dense_cholesky_test.cc
index 42b746e..59ee9f7 100644
--- a/internal/ceres/dense_cholesky_test.cc
+++ b/internal/ceres/dense_cholesky_test.cc
@@ -43,8 +43,7 @@
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 
-namespace ceres {
-namespace internal {
+namespace ceres::internal {
 
 using Param = DenseLinearAlgebraLibraryType;
 
@@ -118,5 +117,4 @@
 
 INSTANTIATE_TEST_SUITE_P(_, DenseCholeskyTest, MakeValues(), ParamInfoToString);
 
-}  // namespace internal
-}  // namespace ceres
+}  // namespace ceres::internal