[OpenMP] Remove compiler warning about unused value

The compiler warns about an unused variable/statement:

    runtime/src/kmp_affinity.cpp:4958:18: warning: statement has no effect [-Wunused-value]
       KA_TRACE(1000, ; {
                      ^
    runtime/src/kmp_debug.h:84:24: note: in definition of macro 'KA_TRACE'
         __kmp_debug_printf x;                                                      \
                            ^

Instead of the unused reference to this function, this patch now calls the function
with an empty string. The call to this function should have no effect.

Patch provided by joachim.protze

Reviewers: jlpeyton, hbae, AndreyChurbanov

Reviewed By: AndreyChurbanov

Tags: #openmp, #ompt

Differential Revision: https://reviews.llvm.org/D56775

llvm-svn: 351323