[clang][ssaf] Add YAML source-edit format (#204491)
Adds the built-in `SourceEditFormat`, registered under the file
extension `yaml`. The writer drives `llvm::yaml::Output` against the
existing `clang::tooling::TranslationUnitReplacements` `MappingTraits`
from `clang/Tooling/ReplacementsYaml.h`, so the resulting document is
byte-for-byte consumable by `clang-apply-replacements`.
Anchored via `SSAFYAMLSourceEditFormatAnchorSource` so static builds
keep the registration.
Assisted-By: Claude Opus 4.7
