[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

llvm-svn: 259651