[Remarks] YAMLRemarkSerializer: StringRef-ize apis to avoid out-of-bounds read footguns (#160397)
In #159759, Tobias identified that because YAML IO `mapRequired`
expected a null-terminated `const char * Key`, we couldn't legally pass
a `StringRef` to it, as that might be length-terminated and not
null-terminated. In this patch, we move all of the YAML IO functions
that accept a `const char *` over to `StringRef`, avoiding that footgun
altogether.