[clang-tidy] argument to operator delete is the address of a local stack variable, which is not memory allocated by 'new'

`clang-tidy` on a project brought this to my attention:
```txt
/usr/lib/gcc/x86_64-redhat-linux/7/../../../../include/c++/7/ext/new_allocator.h:125:2: error: Argument to operator delete is the address of a local stack variable, which is not memory allocated by 'new' [clang-analyzer-cplusplus.NewDelete,-warnings-as-errors]
::operator delete(__p);
^
...
my_private_include_path/include/boost/multiprecision/number.hpp:2005:25: note: Calling 'number::str'
std::string s = r.str(d, os.flags());
^
my_private_include_path/include/boost/multiprecision/number.hpp:808:14: note: Calling 'cpp_int_backend::str'
return m_backend.str(digits, f);
^
my_private_include_path/include/boost/multiprecision/cpp_int.hpp:1920:14: note: Calling 'cpp_int_backend::do_get_string'
return do_get_string(f, trivial_tag());
^
my_private_include_path/include/boost/multiprecision/cpp_int.hpp:1822:11: note: Assuming the condition is false
if ((f & std::ios_base::oct) == std::ios_base::oct)
^
my_private_include_path/include/boost/multiprecision/cpp_int.hpp:1822:7: note: Taking false branch
if ((f & std::ios_base::oct) == std::ios_base::oct)
^
my_private_include_path/include/boost/multiprecision/cpp_int.hpp:1824:16: note: Assuming the condition is false
else if ((f & std::ios_base::hex) == std::ios_base::hex)
^
my_private_include_path/include/boost/multiprecision/cpp_int.hpp:1824:12: note: Taking false branch
else if ((f & std::ios_base::hex) == std::ios_base::hex)
^
my_private_include_path/include/boost/multiprecision/cpp_int.hpp:1830:11: note: 'base' is not equal to 8
if (base == 8 || base == 16)
^
my_private_include_path/include/boost/multiprecision/cpp_int.hpp:1830:11: note: Left side of '||' is false
my_private_include_path/include/boost/multiprecision/cpp_int.hpp:1830:24: note: 'base' is not equal to 16
if (base == 8 || base == 16)
^
my_private_include_path/include/boost/multiprecision/cpp_int.hpp:1830:7: note: Taking false branch
if (base == 8 || base == 16)
^
my_private_include_path/include/boost/multiprecision/cpp_int.hpp:1876:10: note: Taking false branch
if (t.sign())
^
my_private_include_path/include/boost/multiprecision/cpp_int.hpp:1881:14: note: Assuming the condition is false
if (this->size() == 1)
^
my_private_include_path/include/boost/multiprecision/cpp_int.hpp:1881:10: note: Taking false branch
if (this->size() == 1)
^
my_private_include_path/include/boost/multiprecision/cpp_int.hpp:1889:13: note: Loop condition is false. Execution continues on line 364
while (eval_get_sign(t) != 0)
^
my_private_include_path/include/boost/multiprecision/cpp_int.hpp:1904:10: note: Calling implicit destructor for 'cpp_int_backend<0, 0, boost::multiprecision::signed_magnitude, boost::multiprecision::unchecked, std::allocator>'
}
^
my_private_include_path/include/boost/multiprecision/cpp_int.hpp:1904:10: note: Calling '~cpp_int_base'
my_private_include_path/include/boost/multiprecision/cpp_int.hpp:364:11: note: Assuming field 'm_internal' is false
if (!m_internal)
^
my_private_include_path/include/boost/multiprecision/cpp_int.hpp:364:7: note: Taking true branch
if (!m_internal)
^
my_private_include_path/include/boost/multiprecision/cpp_int.hpp:365:10: note: Calling 'new_allocator::deallocate'
allocator().deallocate(limbs(), capacity());
^
/usr/lib/gcc/x86_64-redhat-linux/7/../../../../include/c++/7/ext/new_allocator.h:125:2: note: Argument to operator delete is the address of a local stack variable, which is not memory allocated by 'new'
::operator delete(__p);
^
```
I am not sure but it seems that `clang-tidy` misinterprets the call to the custom allocator `scoped_shared_storage`.

Start with boost/multiprecision/number.hpp around line 2005 and boost/multiprecision/cpp_int.hpp around lines 1822-1904, then inspect the allocator path at line 365 and the scoped_shared_storage implementation. Reproduce the clang-tidy diagnostic with the reported string conversion, determine whether the warning is valid, and document or test the resulting behavior.

Written by the indexing model from the issue text.