raw_ostream: add operator<< overload for std::error_code

Summary:
The main motivation for this is unit tests, which contain a large macro
for pretty-printing std::error_code, and this macro is duplicated in
every file that needs to do this. However, the functionality may be
useful elsewhere too.

In this patch I have reimplemented the existing ASSERT_NO_ERROR macros
to reuse the new functionality, but I have kept the macro (as a
one-liner) as it is slightly more readable than ASSERT_EQ(...,
std::error_code()).

Reviewers: sammccall, ilya-biryukov

Subscribers: zturner, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65643

llvm-svn: 368849