Fix type detection for 'char' variables

A char can have signed and unsigned encoding but previously lldb always
assumed it is signed. This CL adds a logic to detect the encoding of
'char' types based on the default encoding on the target architecture.
It fixes variable printing and expression evaluation on architectures
where 'char' is signed by default.

Differential revision: http://reviews.llvm.org/D8636

llvm-svn: 233682