[OpenMP] Rename last file to cpp and remove LIBOMP_CFLAGS

All other files are already C++ and the build system has always
passed '-x c++' for C files, effectively compiling them as C++.

To stay warning free we need one fix in ittnotify_static.{c,cpp}:
The variable dll_path can be written to, so it must not be const.
GCC complained with -Wcast-qual and I think it's right.

Differential Revision: https://reviews.llvm.org/D65285

llvm-svn: 367343