[BUILD] llvm15 bootstrap · Pull Request !222 · OpenHarmony/third_party_llvm-project

### Issue
https://gitee.com/openharmony/third_party_llvm-project/issues/I5KZDR

### Reason (Purpose, Solved Problem)
Build scripts updated to use prebuilts/clang/ohos/linux-x86-64 with clang-15.0.4 instead of clang-10.0.1+llvm

### Description
Changes:
   1. env_prepare.sh updated to download and unpack llvm15-based build
https://mirrors.huaweicloud.com/openharmony/compiler/clang/15.0.4-025192/linux/clang_linux-x86_64-025192-20230330.tar.bz2
   2. env_prepare.sh updated to detect clang version (as a suffix in dir name)
and to write that version to .../prebuilts_clang_version.py
   3. env_prepare.sh now checks if some archives are already in
download_packages and, if so, does not download them again but
instead uses local copies
   4. build.py imports file with version and sets  self.CLANG_VERSION to it
   5. .gitigore added to not to commit auto-generated file with version

### Test (Local test)
tested locally

### Need to cherry-pick to release (eg：monthly_20221018, OpenHarmony-3.2-Beta5 ...) branch？

- [ ] yes，branch list:
- [x] no

why:

类型	指派人员	状态
审查	hhj Pavel Kosov 已审查通过	已完成 (1/1人)
测试	openharmony_ci 已测试通过	已完成 (1/1人)

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