[llvm] [YAML] fix output incorrect format for block scalar string (PR #132897)

[Congcong Cai via llvm-commits]

HerrCai0907 wrote:

> You can reopen pull requests

I try to reopen it but failed.

> Test has merge conflict

I will reland previous PR and re-run the test

https://github.com/llvm/llvm-project/pull/132897