[z.OS] fix strnlen() usage in Context.cpp available on z/OS (#160339)
On z/OS `strnlen()` is not available by default so we use the wrapper
header `zOSSupport.h` to make it visible. This will fix the following
error:
```
clang/lib/AST/ByteCode/Context.cpp:250:16: error: use of undeclared identifier 'strnlen'
250 | Result = strnlen(reinterpret_cast<const char *>(Ptr.getRawAddress()), N);
| ^~~~~~~
1 error generated.
```