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/numeric_diff_first_order_function_test.cc

@@ -36,8 +36,7 @@
 #include "ceres/first_order_function.h"
 #include "gtest/gtest.h"
 
-namespace ceres {
-namespace internal {
+namespace ceres::internal {
 
 class QuadraticCostFunctor {
  public:
@@ -75,5 +74,4 @@ TEST(NumericDiffFirstOrderFunction, BilinearDifferentiationTest) {
   EXPECT_NEAR(gradient[3], parameters[2], kTolerance);
 }
 
-}  // namespace internal
-}  // namespace ceres
+}  // namespace ceres::internal