Replace string_view with string.

The use of string_view causes problems with absl::StrCat
in certain compilation configurations.

Change-Id: I1beda82692378bf9cf378a7d42df46de8491d1c9

internal/ceres/solver_utils.cc

diff --git a/internal/ceres/solver_utils.cc b/internal/ceres/solver_utils.cc
index ac22210..4162b46 100644
--- a/internal/ceres/solver_utils.cc
+++ b/internal/ceres/solver_utils.cc
@@ -91,6 +91,6 @@
   ;
 // clang-format on
 
-std::string_view VersionString() noexcept { return kVersion; }
+std::string VersionString() { return kVersion; }
 
 }  // namespace ceres::internal

internal/ceres/solver_utils.h

diff --git a/internal/ceres/solver_utils.h b/internal/ceres/solver_utils.h
index ff5e280..60bca91 100644
--- a/internal/ceres/solver_utils.h
+++ b/internal/ceres/solver_utils.h
@@ -32,7 +32,7 @@
 #define CERES_INTERNAL_SOLVER_UTILS_H_
 
 #include <algorithm>
-#include <string_view>
+#include <string>
 
 #include "ceres/internal/disable_warnings.h"
 #include "ceres/internal/export.h"
@@ -59,8 +59,7 @@
   }
 }
 
-CERES_NO_EXPORT
-std::string_view VersionString() noexcept;
+CERES_NO_EXPORT std::string VersionString();
 
 }  // namespace ceres::internal