[llvm-branch-commits] [Remarks] YAMLRemarkSerializer: Fix StringRef out-of-bounds read (PR #159759)

[Tobias Stadler via llvm-branch-commits]

https://github.com/tobias-stadler created https://github.com/llvm/llvm-project/pull/159759

YAML IO `mapRequired` expects a null-terminated `const char *` Key, so
we can't legally pass a StringRef to it. We should add StringRef Key
support to YAML IO, but for now just copy the key into a correctly
null-terminated string.