[libc++][test] Clean up code in GenerateInput.h for benchmark testing (#115560)
This PR refines the code in `GenerateInput.h` used for benchmark testing
by implementing the following changes:
- Replaced all unqualified usages of `size_t` with `std::size_t`.
- Removed unnecessary curly braces `{}` from for loops that contain
simple single-statement bodies, in accordance with LLVM coding
standards.