clang-rename: let -force handle multiple renames

Summary:
The use case is that renaming multiple symbols in a large enough codebase is
much faster if all of these can be done with a single invocation, but
there will be multiple translation units where one or more symbols are
not found.

Old behavior was to exit with an error (default) or exit without
reporting an error (-force). New behavior is that -force results in a
best-effort rename: rename symbols which are found and just ignore the
rest.

The existing help for -force sort of already implies this behavior.

Reviewers: cfe-commits, klimek, arphaman

Reviewed By: klimek

Differential Revision: https://reviews.llvm.org/D37634

llvm-svn: 312942