Fix VASprintfTest.cpp for Darwin, add checks

Summary:
The EncodingError test ensures that trying to encode a multibyte wchar
with a given codepage fails. If setlocale() fails, the encoding is
performed using the current locale, which may or may not fail.

This patch asserts that both setlocale() operations are successful, as
well as falling back to a widely available unibyte encoding for
non-Windows systems.

<rdar://problem/33782806>

Reviewers: zturner, labath, lhames

Reviewed By: zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D36496

llvm-svn: 310499