This fixes several test case failure on s390x caused by the fact that
on this platform, the default "char" type is unsigned.
- In ClangASTContext::GetBuiltinTypeForEncodingAndBitSize we should return an explicit *signed* char type for encoding eEncodingSint and bit size 8, instead of the default platform char type (which may be unsigned). This fix matches existing code in ClangASTContext::GetIntTypeFromBitSize, and fixes the TestClangASTContext.TestBuiltinTypeForEncodingAndBitSize unit test case.
- The test/expression_command/char/TestExprsChar.py test case is known to fail on platforms defaulting to unsigned char (pr23069), and just needs to be xfailed on s390x like on arm.
- The test/functionalities/watchpoint/watchpoint_on_vectors/main.c test case defines a vector of "char" and implicitly assumes to be signed. Use an explicit "signed char" instead.