clang-format: Add initial #include sorting capabilities.

clang-format: Add initial #include sorting capabilities.

To implement this nicely, add a function that merges two sets of
replacements that are meant to be done in sequence. This functionality
will also be useful for other applications, e.g. formatting the result
of clang-tidy fixes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@248367 91177308-0d34-0410-b5e6-96231b3b80d8