[OpenMP] Fix warnings about unused expressions when OMPT_LOOP_DISPATCH is a no-op. NFC.

This fixes warnings like these:

../runtime/src/kmp_dispatch.cpp:2159:24: warning: left operand of comma operator has no effect [-Wunused-value]
    OMPT_LOOP_DISPATCH(*p_lb, *p_ub, pr->u.p.st, status);
                       ^~~~~
../runtime/src/kmp_dispatch.cpp:2159:31: warning: left operand of comma operator has no effect [-Wunused-value]
    OMPT_LOOP_DISPATCH(*p_lb, *p_ub, pr->u.p.st, status);
                              ^~~~~
../runtime/src/kmp_dispatch.cpp:2159:46: warning: left operand of comma operator has no effect [-Wunused-value]
    OMPT_LOOP_DISPATCH(*p_lb, *p_ub, pr->u.p.st, status);
                                     ~~~~~~~ ^~
../runtime/src/kmp_dispatch.cpp:2159:50: warning: expression result unused [-Wunused-value]
    OMPT_LOOP_DISPATCH(*p_lb, *p_ub, pr->u.p.st, status);
                                                 ^~~~~~