Change __INTx_TYPE__ to be always signed. This changes the value for
char-based types from "char" to "signed char". Adjust stdint.h to use
__INTx_TYPE__ directly without prefixing it with signed and to use
__UINTx_TYPE__ for unsigned ones.

The value of __INTx_TYPE__ now matches GCC.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214119 91177308-0d34-0410-b5e6-96231b3b80d8