cpp11-migrate: FileOverrides/Transform refactoring.

This commit include the following changes:
 - SourceOverrides is now a class
   - it simplifies the usage for the Transform class, since now the
     replacements can be applied directly to the file overrides with
     SourceOverrides::applyReplacements().
   - it contains a method applyRewrites() which was previously named
     collectResults() in Transform.cpp. The method has been "optimized"
     a bit to re-use the allocated buffer (std::string::clear() is called).
   - since the class has some logic it's now unit tested
 - Now FileOverrides is a class (not a std::map typedef) and store pointers
   to the SourceOverrides. The reason is that the SourceOverrides can't be
   copied anymore (which was already something to avoid since it's can be a
   quite large object).

Author: Guillaume Papin <guillaume.papin@epitech.eu>

Differential Revision: http://llvm-reviews.chandlerc.com/D1122