[OpenMP] Enable the IDE support for the device runtime

Currently the device runtime is built as a custom target, which will not be included
in the compile commands. Those language servers using compile commands cannot
handle device runtime correctly.

In this patch, when `CMAKE_EXPORT_COMPILE_COMMANDS` is turned on, dummy
targets that will be excluded from all will be added. Those targets will not be
built or installed if we just simply do `make` or `make install`, but their
compilation will be included in the compile commands.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D148870