[OpenCL] Support pipe keyword in C++ mode

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@361382 91177308-0d34-0410-b5e6-96231b3b80d8