[libFuzzer] Remove too aggressive static_assert in FuzzedDataProvider.

[Dor1s]

Summary

http://lab.llvm.org:8011/builders/clang-cmake-aarch64-full/builds/31

error: static_assert failed due to requirement
'std::numeric_limits<char>::is_signed' "Destination type must be
signed."

static_assert(std::numeric_limits<TS>::is_signed,
^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/home/buildslave/buildslave/clang-cmake-aarch64-full/llvm/projects/compiler-rt/lib/fuzzer/utils/FuzzedDataProvider.h:126:19:
note: in instantiation of function template specialization
'FuzzedDataProvider::ConvertUnsignedToSigned<char, unsigned char>'
requested here

char next = ConvertUnsignedToSigned<char>(data_ptr_[0]);
            ^

1 error generated.

[Dor1s]

Self-approval for a minor fix.

compiler-rt/trunk/lib/fuzzer/utils/FuzzedDataProvider.h

std::vector<T> ConsumeBytes(size_t size, size_t num_bytes_to_consume) {
     result.shrink_to_fit();
     return result;
   }
 
   template <typename TS, typename TU> TS ConvertUnsignedToSigned(TU value) {
     static_assert(sizeof(TS) == sizeof(TU), "Incompatible data types.");
     static_assert(!std::numeric_limits<TU>::is_signed,
                   "Source type must be unsigned.");
-    static_assert(std::numeric_limits<TS>::is_signed,
-                  "Destination type must be signed.");
 
     // TODO(Dor1s): change to `if constexpr` once C++17 becomes mainstream.
     if (std::numeric_limits<TS>::is_modulo)
       return static_cast<TS>(value);
 
     // Avoid using implementation-defined unsigned to signer conversions.
     // To learn more, see https://stackoverflow.com/questions/13150449.
     if (value <= std::numeric_limits<TS>::max())