[OpenCL] Support pipe keyword in C++ mode

Support the OpenCL C pipe feature in C++ for OpenCL mode, to preserve
backwards compatibility with OpenCL C.

Various changes had to be made in Parse and Sema to enable
pipe-specific diagnostics, so enable a SemaOpenCL test for C++.

Differential Revision: https://reviews.llvm.org/D62181

llvm-svn: 361382