[Thumb] Clang thinks "char" is signed when using a thumb triple

'char' is unsigned on all ARM and Thumb architectures. Clang gets this
right for ARM, and for thumb when using and arm triple and the -mthumb
option, but gets it wrong for thumb triples. This fixes that.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220555 91177308-0d34-0410-b5e6-96231b3b80d8