[clang] [SystemZ][z/OS] Add visibility features for z/OS (eg. _Export, pragma export) (PR #111035)

[Hubert Tong via cfe-commits]

================
@@ -0,0 +1,5 @@
+// REQUIRES: systemz-registered-target
----------------
hubert-reinterpretcast wrote:

Should there be diagnostics (and corresponding testing) for ignored/meaningless uses of `_Export`?

e.g.,
```cpp
typedef int _Export ty;
ty x;
int f(int _Export x);
static int _Export s;
struct S {
  int _Export nonstaticdatamember;
};
void g() {
  int _Export automatic;
}
```

https://github.com/llvm/llvm-project/pull/111035