Fix DoNotOptimize compilation error with partially const structs (Fixes #1997)

[MaciekChoszczyk]

Fixes #1997.

What's wrong?

Passing structs with const members to DoNotOptimize causes compilation errors because the asm constraints (+m, +r) request write access to memory that the compiler considers const.

Fix

Cast the input value to a raw byte array (char(&)[sizeof(Tp)]) to strip const qualifiers for the optimizer. This safely allows write access in the assembly block.

Notes

• const_cast is not removing 'const' from internal struct members.
• std::bit_cast was considered but avoided to maintain compatibility with older C++ standards.
• This approach aligns with the existing workaround used for MSVC, which was missing from the GCC/Clang implementations.