cpp11-migrate: Write header replacements to disk

Header replacements are now written to disk in YAML format for an external tool
to merge. A unique file will be created in the same directory as the header
with all replacements that came from a source file that included the header
file. The YAML file will have:
 - Name of the file
 - Transform ID that generated the replacement
 - Offset
 - Length
 - Replacement text

Any tool reading these replacements should read them using the TransformDocument
struct.

Differential Revision: http://llvm-reviews.chandlerc.com/D1142