[OpenMP] Fix LIBOMP_USE_DEBUGGER=ON build (PR38612)

Summary:
Two things:
1. Those two variables had the wrong sigdness, which was resulting in "sign mismatch in comparison" warning.
2. The whole `kmp_debugger.cpp` wasn't being built, or rather, it was being built as-if `USE_DEBUGGER` was off,
   thus, nothing provided the definition of `__kmp_omp_debug_struct_info`, `__kmp_debugging`.
   Makes sense, because `USE_DEBUGGER` is set in `kmp_config.h`, which is not included explicitly.
   It is included by `kmp.h`, but that one is only included inside of the `#if USE_DEBUGGER` block..
   I *think* this is the only source file with this issue,
   everything else seem to `#include` either `kmp.h` or `kmp_config.h`.
   The alternative solution would be to add `add_compile_options(-include kmp_config.h)` in CMake.

I did verify that `__kmp_omp_debug_struct_info` becomes available with this patch.

Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=38612 | PR38612 ]].

Reviewers: AndreyChurbanov, jlpeyton, Hahnfeld

Reviewed By: jlpeyton

Subscribers: guansong, jfb, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D55783

llvm-svn: 351019