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

internal/ceres/gradient_checking_cost_function_test.cc

@@ -50,8 +50,7 @@
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 
-namespace ceres {
-namespace internal {
+namespace ceres::internal {
 
 using std::vector;
 using testing::_;
@@ -526,5 +525,4 @@ TEST(GradientCheckingProblemImpl, ConstrainedProblemBoundsArePropagated) {
   }
 }
 
-}  // namespace internal
-}  // namespace ceres
+}  // namespace ceres::internal