Feature: Macros for identifying the wide and narrow execution string literal encoding

gcc/c-family
	* c-cppbuiltin.c (c_cpp_builtins): Add predefined
	{__GNUC_EXECUTION_CHARSET_NAME} and
	_WIDE_EXECUTION_CHARSET_NAME} macros.

gcc/
	* doc/cpp.texi: Document new macros.

gcc/testsuite/
	* c-c++-common/cpp/wide-narrow-predef-macros.c: New test.

libcpp/
	* charset.c (init_iconv_desc): Initialize "to" and "from" fields.
	* directives.c (cpp_get_narrow_charset_name): New function.
	(cpp_get_wide_charset_name): Likewise.
	* include/cpplib.h (cpp_get_narrow_charset_name): Prototype.
	(cpp_get_wide_charset_name): Likewise.
	* internal.h (cset_converter): Add "to" and "from" fields.