Add String frontend · Pull Request !306 · OpenHarmony/third_party_llvm-project

### Issue  
https://gitee.com/openharmony/third_party_llvm-project/issues/I7XXT4

### Description

Add synthetic frontend for std::string (std::wstring, std::u16string, std::u32string).  
Add new API method to SyntheticFrontend class - SetValueFromCString - sometimes we want our frontend to decide how to set values. (was suggested here: https://discourse.llvm.org/t/clarify-hostaddress-loadaddress-logic/72175/3)  
Four tests:  
`lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx/map/TestDataFormatterLibccMap.py`  
`lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx/shared_ptr/TestDataFormatterLibcxxSharedPtr.py`  
`lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx/unique_ptr/TestDataFormatterLibcxxUniquePtr.py`  
`lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/multimap/TestDataFormatterGenericMultiMap.py`  
Were depend on the way how objects are printed in lldb, and since string currently have synthetic child - they need to be updated also.  
(See comment here: https://gitee.com/openharmony/third_party_llvm-project/blob/master/lldb/source/DataFormatters/FormatManager.cpp#L510)

### Test (Local test)

### 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	进行中 (0/1人)
测试	openharmony_ci	进行中 (0/1人)

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