clang-rename: add new -force option

Summary:
    The use-case is when renaming a widely used name, like a lower-level
    class in a codebase and clang-rename is simply invoked for each
    translation unit based on the compile database. In this case it's not
    interesting to show errors: not finding the symbol means there is
    simply nothing to do.

Reviewers: klimek

Reviewed By: klimek

Differential Revision: https://reviews.llvm.org/D32403

llvm-svn: 304534