[clang] [z/OS][Clang] Add wrapper headers to avoid macro name conflicts (PR #204472)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-solaris11-sparcv9` running on `solaris11-sparcv9` while building `clang` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/13/builds/13811

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 5 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'Clang :: Headers/zos-guard.c' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
/opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/stage1/bin/clang -c  --target=s390x-ibm-zos /opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/llvm/clang/test/Headers/zos-guard.c -mzos-sys-include=/opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/llvm/clang/test/Headers/Inputs/zos/usr/include -Xclang -verify
# executed command: /opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/stage1/bin/clang -c --target=s390x-ibm-zos /opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/llvm/clang/test/Headers/zos-guard.c -mzos-sys-include=/opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/llvm/clang/test/Headers/Inputs/zos/usr/include -Xclang -verify
# .---command stderr------------
# | error: 'expected-error' diagnostics seen but not expected: 
# |   (frontend): unable to create target: 'No available targets are compatible with triple "s390x-ibm-zos"'
# | 1 error generated.
# `-----------------------------
# error: command failed with exit status: 1

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/204472