[OpenMP]Fix trivial build failure in MacOS

[Leporacanthicus]

Summary

MacOS build of LLVM with OpenMP enabled fails with an error
that it doesn't know what std::abs is. Fix by including <cmath>
so that the relevant function declaration is included.

No functional change intended.

[tianshilei1992]

Hmm, I'm not sure if we want to include <cmath> here because libomp definitely doesn't depend on libm. We might want to deal with it in https://reviews.llvm.org/D149010 instead.

[Leporacanthicus]

In D150687#4346761, @tianshilei1992 wrote:

Hmm, I'm not sure if we want to include <cmath> here because libomp definitely doesn't depend on libm. We might want to deal with it in https://reviews.llvm.org/D149010 instead.

I guess we could include cstdlib instead, would that be better. If you think that's better, let me know, happy to update the patch.

The other alternative would be to "not use std::abs" - but to me that doesn't seem like a great solution. [Interestingly, the file kmp_stats.cpp, line 181 does call std::fabs, but I guess that is done inline, so it doesn't actually use libm anyway] - and includes <cmath> - maybe kmp_stats.cpp is "special" here? I'm happy for someone else to fix this - my main reason for posting this patch was to help some colleagues of mine move on - which they are now able to do. If another fix is done elsewhere, I'm happy to abandon this patch.

[tianshilei1992]

Yeah, it is fine as long as the function can be inlined. Thanks for pointing to openmp/runtime/src/kmp_stats.cpp. I didn't notice that. We generally avoid pulling dependences for libomp. I think we can go with it for now. If anything comes up in the future, we can always revisit it.

openmp/runtime/src/kmp_collapse.cpp

 #include "kmp.h"
 #include "kmp_error.h"
 #include "kmp_i18n.h"
 #include "kmp_itt.h"
 #include "kmp_stats.h"
 #include "kmp_str.h"
 #include "kmp_collapse.h"
 
+#include <cmath>
+
 #if OMPT_SUPPORT
 #include "ompt-specific.h"
 #endif
 
 // OMPTODO: different style of comments (see kmp_sched)
 // OMPTODO: OMPT/OMPD
 
 //----------------------------------------------------------------------------