[clang] [z.OS] fix strnlen() usage in Context.cpp available on z/OS (PR #160339)

[Sean Perry via cfe-commits]

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@@ -18,6 +18,7 @@
 #include "clang/AST/ASTLambda.h"
 #include "clang/AST/Expr.h"
 #include "clang/Basic/TargetInfo.h"
+#include "llvm/Support/SystemZ/zOSSupport.h"
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perry-ca wrote:

I think this will get deleted again.  The compiles on other platforms are going to say this header is empty and it will get removed again.  

What I was thinking should do is:
1. create a wrapper header dir that is just for building llvm.  This can live in the llvm repo (or we prereq the zoslib repo which has this already)
  - create a stdlib.h that has the strnlen() in it 
2. add a -isystem option to include this wrapper.

I'm good with the change.  Just worried it will get undone.

https://github.com/llvm/llvm-project/pull/160339