Avoid -Wdangling-reference false positives in locale helpers

[viordash]

GCC 13/14 warns in two locale helpers when a precompiled header is used:

include/fmt/format-inl.h:132: warning: possibly dangling reference to a temporary
  [-Wdangling-reference]
include/fmt/xchar.h:49: same

It is a false positive: locale_ref::get<std::locale>() returns a copy, and the
reference returned by std::use_facet stays valid as long as any copy of that locale
exists. GCC only warns when the declaration comes from a precompiled header; with
<locale> included normally it stays quiet. Reproducible with plain <locale>,
no fmt involved:

// t.cpp
#include <locale>
template <typename Char> auto f(const std::locale& l) -> Char {
  auto&& facet = std::use_facet<std::numpunct<Char>>(std::locale(l));
  return facet.decimal_point();
}
template char f<char>(const std::locale&);

// pch.h
#include <locale>

g++ -Wdangling-reference -c t.cpp                    # quiet
g++ -x c++-header pch.h -o pch.h.gch
g++ -Wdangling-reference -include pch.h -c t.cpp     # warns

(GCC 13.4; -Wdangling-reference comes with -Wextra there)

This patch binds the locale copy to a named variable at both call sites, so no reference
is bound to a temporary. Same single copy, no behaviour change, and the lifetime becomes
explicit instead of relying on the "valid while any copy exists" guarantee.

Checked with GCC 13.4: warnings gone when a PCH is in use, ctest passes 21/21, and
locale-aware output is unchanged

Originally filed against spdlog, which vendors these files (gabime/spdlog#3638), and
closed there with a pointer to this repository.

[vitaut]

Could you provide a godbolt CMake-project (since it requires PCH) repro?

[viordash]

Here is a CMake project repro: https://godbolt.org/z/vfcfWThT7

/opt/compiler-explorer/libs/fmt/trunk/include/fmt/format-inl.h:132:10: warning:
  possibly dangling reference to a temporary [-Wdangling-reference]

Removing the target_precompile_headers() line makes it quiet.