Fix an unused variable warning in `Errors.cpp`.
Use brace initialization syntax in `SymbolInfo.cpp` rather than using
a constructor call.
rdar://130992923
Prefer `ZeroMemory()` to `::memset()` here.
Use `sizeof(*wszBuffer)` instead of `sizeof(WCHAR)`, just in case.
Don't use `*this=other`, because that motivates tests around `::free()`,
but instead pull the shared code out into some private functions.
Also, fix `SymbolInfo()` to initialize the pointer members.
rdar://130992923
We were calling `SymInitialize()` multiple times, which is wrong, which
was making the `SymbolInfo::lookup()` call fail. Also, we weren't
fetching the module names, so we should do that too.
rdar://130992923
