[clang-tidy] Add option to export fixes to run-clang-tidy.py

This patch adds the option to keep the list of proposed fixes even though they
should not be applied.

This allows to detect possible fixes using the parallelised run-clang-tidy.py
and apply them using clang-apply-replacements at a later time.
Essentially the patch causes the individual temporary yaml files by the parallel
clang-tidy instances to be merged into one user-defined file.

Patch by Michael F. Herbst!
Differential revision: https://reviews.llvm.org/D31326

llvm-svn: 308726