Fix constexpr for char traits tests #432

Job	Run time
Syntax Check - Clang C++03 Linux No STL (ubuntu-22.04)	25s
Syntax Check - Clang C++03 Linux STL (ubuntu-22.04)	23s
Syntax Check - Clang C++11 Linux No STL (ubuntu-22.04)	41s
Syntax Check - Clang C++11 Linux No STL Force C++03 (ubuntu-22.04)	34s
Syntax Check - Clang C++11 Linux STL (ubuntu-22.04)	43s
Syntax Check - Clang C++11 Linux STL Force C++03 (ubuntu-22.04)	36s
Syntax Check - Clang C++14 Linux No STL (ubuntu-22.04)	45s
Syntax Check - Clang C++14 Linux No STL Force C++03 (ubuntu-22.04)	39s
Syntax Check - Clang C++14 Linux STL (ubuntu-22.04)	56s
Syntax Check - Clang C++14 Linux STL Force C++03 (ubuntu-22.04)	40s
Syntax Check - Clang C++17 Linux No STL (ubuntu-22.04)	44s
Syntax Check - Clang C++17 Linux No STL Force C++03 (ubuntu-22.04)	45s
Syntax Check - Clang C++17 Linux STL (ubuntu-22.04)	55s
Syntax Check - Clang C++17 Linux STL Force C++03 (ubuntu-22.04)	1m 3s
Syntax Check - Clang C++20 Linux No STL (ubuntu-22.04)	1m 1s
Syntax Check - Clang C++20 Linux No STL Force C++03 (ubuntu-22.04)	1m 5s
Syntax Check - Clang C++20 Linux STL (ubuntu-22.04)	1m 23s
Syntax Check - Clang C++20 Linux STL Force C++03 (ubuntu-22.04)	1m 14s
	14m 32s

Provide feedback