[OpenCL] Adding reserved operator logical xor for OpenCL

This patch adds the reserved operator ^^ when compiling for OpenCL (spec v1.1 s6.3.g),
which results in a more meaningful error message.

Patch by Neil Hickey!

Review: http://reviews.llvm.org/D13280

M    test/SemaOpenCL/unsupported.cl
M    include/clang/Basic/TokenKinds.def
M    include/clang/Basic/DiagnosticParseKinds.td
M    lib/Basic/OperatorPrecedence.cpp
M    lib/Lex/Lexer.cpp
M    lib/Parse/ParseExpr.cpp


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@259651 91177308-0d34-0410-b5e6-96231b3b80d8