libcpp: Update cpp_wcwidth() to Unicode 15

Updates cpp_wcwidth() to Unicode 15, following the procedure in
contrib/unicode/README mechanically without incident.

contrib/ChangeLog:

	* unicode/DerivedCoreProperties.txt: Update to Unicode 15.
	* unicode/DerivedNormalizationProps.txt: Likewise.
	* unicode/EastAsianWidth.txt: Likwise.
	* unicode/PropList.txt: Likewise.
	* unicode/README: Likewise.
	* unicode/UnicodeData.txt: Likewise.

libcpp/ChangeLog:

	* generated_cpp_wcwidth.h: Regenerated for Unicode 15.