Otherwise breaks on compilers without noexcept support

The intel-17 and intel-18 compilers seem to require that `u` be `const`:
```
/src/fmt/format.h(226): warning #437: reference to local variable of enclosing function is not allowed
        char data[sizeof(u)];
```
If `u` is declared as `const auto u =1u` instead of just `auto u=1u`, the file compiles with no warnings.

The `std::is_base_of<T,U>()` and `std::is_reference<T>()` member functions were added in C++14.  To maintain C++11 compatibility, use the `::value` instead.

Current code fails on intel-17 and other compilers if using strict C++11

The nvcc compiler (at least up to 9.2) defines `__SIZEOF_INT128__`, but doesn't support 128-bit integers on device code:
```
error: "fmt::v6::format_arg_store<fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::internal::buffer<char>>, char>, const char *, int, const char *>" contains a 128-bit integer, which is not supported in device code
```

…hanks Tim Song)

The `operator()` member function of `std::is_same` was added in C++14.  For C++11, the `::value` needs to be used instead.

## Problem

In the case of an existing `fmt` namespace (in my project this looks like `Project::fmt`) it is possible to get a namespace clash in debug builds (MSVC 2017)

## Proposed Solution

When referencing `fmt` internally, be explicit that it is relative to the global namespace using `::fmt`

If `FMT_STATIC_THOUSANDS_SEPARATOR` defined, then locale is not included or defined, so this call will be unresolved.  I think this is the correct fix based on the code in `format-inl.h` and `format.h`

* Fix formatting chrono durations to wide strings

* Make format buffers const correct

* Add FormatWide chrono test case

* Fix incorrect wide encoding of 'µs'
I think might be a source file encoding issue, so I used \u00B5 instead.

* Update FormatWide test to use proper encoding of µs

* Revert changes to format_localized's parameters

* Use different overload of `std::time_put<T>::put` to avoid needing a format string

* Use utf8_to_utf16 instead of having redundant overloads of get_units

* Revert some minor changes

* Remove FMT_CONSTEXPR from expression

This should hopefully fix compilation on VS <2019

* Make suggested changes from code review

* Run clang-format on chrono.h

* Make sure unit isn't null before constructing a string_view from it