Retry^2 "[llvm-profdata] Add option to ingest filepaths from a file"

Changes since the initial commit:
- Normalize file paths read from the file to prevent Windows path
  separators from escaping parts of the path.
- Since we need to store the normalized file paths in WeightedFile,
  don't do tricky things to keep the source MemoryBuffer alive.
- Don't use list-initialization for a std::string in WeightedFile.

Differential Revision: http://reviews.llvm.org/D20980

llvm-svn: 271953