[llvm] [ThinLTO] Use StringRef instead of std::string (PR #97963)

[via llvm-commits]

luxufan wrote:

> Who is the owner of the string? Can you make sure that there is one in the bitcode reader case and also document at the structure definition who that owner is?

In the bitcode reader, the owner is the [string table]( https://github.com/llvm/llvm-project/blob/a5bfe20f6fd8957f870f8ffcf836d7737864e063/llvm/lib/Bitcode/Reader/BitcodeReader.cpp#L7499) of the bitcode module

In the ll parser, the owner is the index itself since this patch uses Index->saveString to save the string in the index.

https://github.com/llvm/llvm-project/pull/97963