[libFuzzer] Remove too aggressive static_assert in FuzzedDataProvider.

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.

Reviewers: Dor1s

Reviewed By: Dor1s

Subscribers: javed.absar, kristof.beyls, delcypher, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D63553

llvm-svn: 363805