[OpenCL] Restrict various keywords in OpenCL C++ mode

Restrict the following keywords in the OpenCL C++ language mode,
according to Sections 2.2 & 2.9 of the OpenCL C++ 1.0 Specification.

 - dynamic_cast
 - typeid
 - register (already restricted in OpenCL C, update the diagnostic)
 - thread_local
 - exceptions (try/catch/throw)
 - access qualifiers read_only, write_only, read_write

Support the `__global`, `__local`, `__constant`, `__private`, and
`__generic` keywords in OpenCL C++.  Leave the unprefixed address
space qualifiers such as global available, i.e., do not mark them as
reserved keywords in OpenCL C++.  libclcxx provides explicit address
space pointer classes such as `global_ptr` and `global<T>` that are
implemented using the `__`-prefixed qualifiers.

Differential Revision: https://reviews.llvm.org/D46022

llvm-svn: 331874