## 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`

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

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
```

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 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.

Otherwise breaks on compilers without noexcept support

Otherwise, Google Test will insist on inserting 'char8_t' NTBS into 'char' streams, but basic_ostream<char>::operator<< overloads taking 'char8_t' arguments are defined as deleted by P1423.
Handling individual 'char8_t's is done inline.

This fixes the compilation errors seen in C++20 mode beginning with VS2019 Update 2.
Signed-off-by: Daniela Engert <dani@ngrt.de>