libstdc++: Improve std::regex_error::what() strings

This replaces the vague "regex_error" for std::regex_error::what() with
a string that corresponds to the error_type enum passed to the
constructor. This allows us to remove many of the strings passed to
__throw_regex_error, because the default string is at least as good.
When a string argument to __throw_regex_error is kept it should add some
context-specific detail absent from the default string.

Also remove full stops (periods) from the end of those strings, to make
it easier to include them in logs and other output. I've left them
starting with an upper-case letter, which is consistent with strerror
output for (at least) Glibc, Solaris and BSD. I'm ambivalent whether
that's the right choice.

This also adds the missing noreturn attribute to __throw_regex_error.

libstdc++-v3/ChangeLog:

	* include/bits/regex_compiler.tcc: Adjust all calls to
	__throw_regex_error.
	* include/bits/regex_error.h (__throw_regex_error): Add noreturn
	attribute.
	* include/bits/regex_scanner.tcc: Likewise.
	* src/c++11/regex.cc (desc): New helper function.
	(regex_error::regex_error(error_type)): Use desc to get a string
	corresponding to the error code.

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