[Libomptarget][NFC] Remove constexpr to hide warnings

Currently whenever we compile the device runtime we get the following
'Mapping.cpp:32:32: warning: inline function '_OMP::impl::getGridValue'
is not defined [-Wundefined-inline]' warning. This can be silenced by
removing the constexpr attribute for this function. Doing this doesn't
change the generated bitcode at all but prevents the screen from getting
filled with warnings whenver we build the runtime.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D119747