Make ArgMap::init not explicitly instantiated

This will remove one more warning that can't be stopped from a calling CMakeLists. This possibly should be set to new at some point (CMake 3.3 behavior would be different / better), as well as 0048.

Suppress warning about missing noreturn attribute

Adding `[[noreturn]]` to `report_unknown_type` suppresses the Clang/GCC `-Wmissing-noreturn` warning:

Clang outputs:

    .../fmt/fmt/format.cc:294:74: warning:
          function 'report_unknown_type' could be declared with
          attribute 'noreturn' [-Wmissing-noreturn]
      ...code, const char *type) {
                                 ^

GCC outputs:

    .../fmt/fmt/format.cc:294:74: warning: function might be candidate for
        attribute 'noreturn' [-Wsuggest-attribute=noreturn]
      ...code, const char *type) {
                                 ^

If a base class can be formatted, it is assumed
that the hierarchy can be formatted from the base
class. The idiom is not uncommon with ostreams.

Similar to the Pullrequest #539, `_SECURE_SCL` caused the same  `warning C4574: '_SECURE_SCL' is defined to be '0': did you mean to use '#if _SECURE_SCL'`.  `_SECURE_SCL` is defined in the `MSVC/14.10.25017/include/yvals.h` by Microsoft itself

`FMT_USE_USER_DEFINED_LITERALS` macro expands to `defined()` which is undefined behavior.

For the consumer it should not matter if fmt has been added to the
project as subdirectory or via find_package. With the alias targets
the library can be always imported via fmt::fmt.

Each instantiation of ArgArray template uses sizeof operator, which
returns a std::size_t value. GCC 7.1 warns about invalid conversion
(error: conversion to ‘unsigned int’ from ‘long unsigned
int’ may alter its value [-Werror=conversion]).