I'm observing the following warnings when building `ninja check-libc` with clang: ``` [286/3491] Building CXX object projects/libc/src/math/gene...es/libc.src.math.generic.nexttowardf.dir/nexttowardf.cpp.o In file included from /android0/llvm-project/libc/src/math/generic/nexttowardf.cpp:10: /android0/llvm-project/libc/src/__support/FPUtil/ManipulationFunctions.h:191:12: warning: implicit conversion loses floating-point precision: 'long double' to 'cpp::enable_if_t<cpp::is_floating_point_v<float>, float>' (aka 'float') [-Wimplicit-float-conversion] return to; ~~~~~~ ^~ /android0/llvm-project/libc/src/math/generic/nexttowardf.cpp:16:18: note: in instantiation of function template specialization '__llvm_libc_18_0_0_git::fputil::nexttoward<float>' requested here return fputil::nexttoward(x, y); ^ In file included from /android0/llvm-project/libc/src/math/generic/nexttowardf.cpp:10: /android0/llvm-project/libc/src/__support/FPUtil/ManipulationFunctions.h:194:12: warning: implicit conversion loses floating-point precision: 'long double' to 'cpp::enable_if_t<cpp::is_floating_point_v<float>, float>' (aka 'float') [-Wimplicit-float-conversion] return to; ~~~~~~ ^~ ```

For free function, I personally prefer to use SFINAE on the returning type `LIBC_INLINE cpp::enable_if_t<cpp::is_floating_point_v<T>, T> nexttoward(...)`, so that it frees the second template parameter to be used if needed.

Should probably test the exceptions in the tests for nextafter too. Will do that in a separate pr?