[clang] [SystemZ][z/OS] Implement _Export keyword (PR #140944)

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> For reference, the XL documentation for _Export: https://www.ibm.com/docs/en/zos/3.1.0?topic=specifiers-export-function-specifier-c-only

@perry-ca, additional context from https://www.ibm.com/docs/en/zos/3.1.0?topic=qualifiers-export-qualifier-c-only would help to cover the case for non-functions. I suggest to update the PR description to also add that.

https://github.com/llvm/llvm-project/pull/140944