[llvm] [llvm][ItaniumDemangle] Use __LDBL_MANT_DIG__ for configuring demangling of long doubles (PR #135968)

[Michael Buch via llvm-commits]

Michael137 wrote:

libc++ failures look unrelated:
```
llvm-libc++-shared-cfg-in.libcxx/containers/views/mdspan/extents/assert.conversion.pass.cpp
llvm-libc++-shared-cfg-in.libcxx/containers/views/mdspan/extents/assert.ctor_from_array.pass.cpp
llvm-libc++-shared-cfg-in.libcxx/containers/views/mdspan/extents/assert.ctor_from_integral.pass.cpp
llvm-libc++-shared-cfg-in.libcxx/containers/views/mdspan/extents/assert.ctor_from_span.pass.cpp
llvm-libc++-shared-cfg-in.libcxx/containers/views/mdspan/extents/assert.obs.pass.cpp
llvm-libc++-shared-cfg-in.libcxx/containers/views/mdspan/layout_left/assert.conversion.pass.cpp
llvm-libc++-shared-cfg-in.libcxx/containers/views/mdspan/layout_left/assert.ctor.extents.pass.cpp
llvm-libc++-shared-cfg-in.libcxx/containers/views/mdspan/layout_left/assert.ctor.layout_right.pass.cpp
llvm-libc++-shared-cfg-in.libcxx/containers/views/mdspan/layout_left/assert.ctor.layout_stride.pass.cpp
llvm-libc++-shared-cfg-in.libcxx/containers/views/mdspan/layout_right/assert.conversion.pass.cpp
llvm-libc++-shared-cfg-in.libcxx/containers/views/mdspan/layout_right/assert.ctor.extents.pass.cpp
llvm-libc++-shared-cfg-in.libcxx/containers/views/mdspan/layout_right/assert.ctor.layout_left.pass.cpp
llvm-libc++-shared-cfg-in.libcxx/containers/views/mdspan/layout_right/assert.ctor.layout_stride.pass.cpp
llvm-libc++-shared-cfg-in.libcxx/containers/views/mdspan/layout_stride/assert.ctor.extents_array.pass.cpp
llvm-libc++-shared-cfg-in.libcxx/containers/views/mdspan/layout_stride/assert.ctor.extents_span.pass.cpp
```

```
# .---command stderr------------
# | /var/lib/buildkite-agent/builds/linux-56-59b8f5d88-bt7k7-1/llvm-project/github-pull-requests/libcxx/test/libcxx/containers/views/mdspan/layout_right/assert.conversion.pass.cpp:30:29: error: declaration of 'dynamic_extent' must be imported from module 'std.span.fwd' before it is required
# |    30 |   constexpr size_t D = std::dynamic_extent;
# |       |                             ^
# | /var/lib/buildkite-agent/builds/linux-56-59b8f5d88-bt7k7-1/llvm-project/github-pull-requests/build/runtimes/runtimes-bins/libcxx/test-suite-install/include/c++/v1/__fwd/span.h:28:25: note: declaration here is not visible
# |    28 | inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();
# |       |                         ^
# | 1 error generated.
# `-----------------------------
# error: command failed with exit status: 1
```

https://github.com/llvm/llvm-project/pull/135968