[Driver] Handle -fno-signed-char and -fno-unsigned-char

GCC maps -fno-unsigned-char to -fsigned-char and -fno-signed-char to
-funsigned-char.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@238105 91177308-0d34-0410-b5e6-96231b3b80d8