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/solver_utils.cc

@@ -40,8 +40,7 @@
 #include "cuda_runtime.h"
 #endif  // CERES_NO_CUDA
 
-namespace ceres {
-namespace internal {
+namespace ceres::internal {
 
 // clang-format off
 #define CERES_EIGEN_VERSION                 \
@@ -97,5 +96,4 @@ std::string VersionString() {
   return value;
 }
 
-}  // namespace internal
-}  // namespace ceres
+}  // namespace ceres::internal