std-test fails on clang-cl + libc++

[FatihBAKIR]

test/std-test.cc(333): error: Value of: fmt::format("{:t}", ex)
Expected: starts with "std::system_error: "
  Actual: "std::__1::system_error: message"
test/std-test.cc(343): error: Value of: fmt::format("{:t}", ex)
Expected: starts with "std::filesystem::filesystem_error: "
  Actual: "std::__1::__fs::filesystem::filesystem_error: filesystem error: message"

This happens when building with libc++ instead of the microsoft standard library for windows using clang-cl. The demangling of the type name when FMT_HAS_ABI_CXA_DEMANGLE is not defined (which is the case when building for MSVC targets) does not consider the inline namespaces, and the test fails.

[FatihBAKIR]

Fixing this by copying the name to a temporary string and doing the same transformation in the FMT_HAS_ABI_CXA_DEMANGLE is pretty trivial, but I suspect we want something that avoids allocations here.

[vitaut]

Fixing this by copying the name to a temporary string and doing the same transformation in the FMT_HAS_ABI_CXA_DEMANGLE is pretty trivial ...

Could you submit a PR? I think allocation is fine - it can always be optimized later if anyone is interested.