[clang] [Format] Do not crash on non-null terminated strings (PR #131299)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

I am going with the path of least resistance in this change and I would welcome any concerns or alternative suggestions.
It definitely has trade-offs as we now incur an extra copy, but this seems acceptable for a convenience API that `SourceManagerForFile` aims to be.


https://github.com/llvm/llvm-project/pull/131299