cpp11-migrate: Write header replacements to disk

Another attempt to commit r187204 after windows related problems has
been fixed. Note that changes to this patch reflect the current behavior
of cpp11-migrate.

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 header file
- Name of the source file that included the header file
- Transform ID that generated the replacement
- Offset
- Length
- Replacement text

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

Differential Revision: http://llvm-reviews.chandlerc.com/D1369

llvm-svn: 188274