Fix clang-tidy crash when a single fix is applied on multiple files.

Summary:
tooling::Replacements only holds replacements for a single file, so
this patch makes Fix a map from file paths to tooling::Replacements so that it
can be applied on multiple files.

Reviewers: hokein, alexfh

Subscribers: Prazek, cfe-commits

Differential Revision: https://reviews.llvm.org/D23257

llvm-svn: 278101