libcpp: Use constexpr for _cpp_trigraph_map initialization for C++14

The _cpp_trigraph_map initialization used to be done for C99+ using
designated initializers, but can't be done that way for C++ because
the designated initializer support in C++ as array designators are just
an extension there and don't allow skipping anything nor going backwards.

But, we can get the same effect using C++14 constexpr constructor.
With the following patch we get rid of the runtime initialization
and the array can be in .rodata.

2024-10-07  Jakub Jelinek  <jakub@redhat.com>

	* internal.h (_cpp_trigraph_map_s): New type for C++14 or later.
	(_cpp_trigraph_map_d): New variable for C++14 or later.
	(_cpp_trigraph_map): Define to _cpp_trigraph_map_d.map for C++14 or
	later.
	* init.cc (init_trigraph_map): Define to nothing for C++14 or later.
	(TRIGRAPH_MAP, END, s): Define differently for C++14 or later.