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/array_utils.h

@@ -48,8 +48,7 @@
 #include "ceres/internal/disable_warnings.h"
 #include "ceres/internal/export.h"
 
-namespace ceres {
-namespace internal {
+namespace ceres::internal {
 
 // Fill the array x with an impossible value that the user code is
 // never expected to compute.
@@ -85,8 +84,7 @@ CERES_NO_EXPORT void AppendArrayToString(const int size,
 // [1 0 2 3 0 1 3]
 CERES_NO_EXPORT void MapValuesToContiguousRange(int size, int* array);
 
-}  // namespace internal
-}  // namespace ceres
+}  // namespace ceres::internal
 
 #include "ceres/internal/reenable_warnings.h"