[arcmt] Introduce new '-ccc-arcmt-migrate <path>' ARC migration driver option.

This is a new mode of migration, where we avoid modifying the original files but
we emit temporary files instead.

<path> will be used to keep migration process metadata. Currently the temporary files
that are produced are put in the system's temp directory but we can put them
in the <path> if is necessary.

Also introduce new ARC migration functions in libclang whose only purpose,
currently, is to accept <path> and provide pairs of original file/transformed file
to map from the originals to the files after transformations are applied.

Finally introduce the c-arcmt-test utility that exercises the new libclang functions,
update arcmt-test, and add tests for the whole process.

rdar://9735086.

llvm-svn: 134844

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