[libc++][test] Update some wstring_convert tests for MSVC quirks

Due to MSVC's decision to encode `wchar_t` as UTF-16, it rejects wide
character/string literals that expect a character value greater than
`\xffff`. UTF-16 `wchar_t` is clearly non-conforming, given that the
standard requires wchar_t to be capable of representing all characters
in the supported wide character execution sets, but rejecting e.g.
`\x40003` is a reasonably sane compromise given that encoding choice:
there's an expectation that `\xFOO` produces a single character in the
resulting literal. Consequently `L'\x40003'`/`L"\x40003"` are ill-formed
literals on MSVC. `L'\U00040003'` is a high surrogate (and produces a
warning about ignoring the "second character" in a multi-character
literal), and `L"\U00040003"` is a perfectly-valid `const wchar_t[3]`.

This change updates these tests to use universal-character-names instead
of raw values for the intended character values, which technically makes
them portable even to implementations that don't use a unicode
transformation format encoding for their wide character execution
character set. The two-character literal `L"\u1005e"` is awkward - the
`e` looks like part of the UCN's hex encoding - but necessary to compile
in '03 mode since '03 didn't allow UCNs to be used for members of the
basic execution character set even in character/string literals.

I've also eliminated the extraneous `\x00` "bonus null-terminator" in
some of the string literals which doesn't affect the tested behavior.

I'm sorry about using `*L"\U00040003"` in `conversions.string/to_bytes.pass.cpp`,
but it's correct for platforms with 32-bit wchar_t, *and* doesn't
trigger narrowing warnings as did the prior `CharT(0x40003)`.

Differential Revision: https://reviews.llvm.org/D60950

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@358908 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed