clang-replace: Afford applying replacements in memory

For users of libclangReplace, this patch affords the ability to apply
replacements in memory instead of writing to disk.

Differential Revision: http://llvm-reviews.chandlerc.com/D1519



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@189493 91177308-0d34-0410-b5e6-96231b3b80d8