Fixes for platforms that default to unsigned char

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.

Differential Revision: http://reviews.llvm.org/D18979

llvm-svn: 266309