[libc++] Fix support for multibyte thousands_sep and decimal_point in...

[libc++] Fix support for multibyte thousands_sep and decimal_point in moneypunct_byname and numpunct_byname.

Summary:
The underlying C locales provide the `thousands_sep` and `decimal_point` as strings, possible with more than one character. We currently don't handle this case even for `wchar_t`.

This patch properly converts the mbs -> wide character for `moneypunct_byname<wchar_t>`. For the `moneypunct_byname<char>` case we attempt to narrow the WC and if that fails we also attempt to translate it to some reasonable value. For example we translate U00A0 (non-breaking space) into U0020 (regular space). If none of these conversions succeed then we simply allow the base class to provide a fallback value.


Reviewers: mclow.lists, EricWF

Subscribers: vangyzen, george.burgess.iv, cfe-commits

Differential Revision: https://reviews.llvm.org/D24218

llvm-svn: 289347