[Fuser] Do not attempt to use OpenMP if build without OpenMP support (#51504)
Summary:
Clang from XCode does not support `-fopenmp` option, no need to try to compile with it.
Infer whether OpenMP is supported by checking _OPENMP define.
Also, use clang compiler if host app was compiled with clang rather than gcc.
Fix few range loop warnings and add static_asserts that range loop variables are raw pointers.
This changes makes fuser tests on OS X a bit faster.
Before:
```
% python3 test_jit.py -v TestScript.test_batchnorm_fuser_cpu
Fail to import hypothesis in common_utils, tests are not derandomized
CUDA not available, skipping tests
test_batchnorm_fuser_cpu (__main__.TestScript) ... clang: error: unsupported option '-fopenmp'
clang: error: unsupported option '-fopenmp'
warning: pytorch jit fuser failed to compile with openmp, trying without it...
ok
----------------------------------------------------------------------
Ran 1 test in 0.468s
OK
```
After:
```
% python3 test_jit.py -v TestScript.test_batchnorm_fuser_cpu
Fail to import hypothesis in common_utils, tests are not derandomized
CUDA not available, skipping tests
test_batchnorm_fuser_cpu (__main__.TestScript) ... ok
----------------------------------------------------------------------
Ran 1 test in 0.435s
OK
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51504
Reviewed By: smessmer
Differential Revision: D26186875
Pulled By: malfet
fbshipit-source-id: 930b3bcf543fdfad0f493d687072aaaf5f9e2bfc
