clang-replace: Re-org of file structure

clang-replace is likely to move to clang proper one day soon. To facilitate
that move, renaming files and directory structure layout to ease transition for
users of clang-replace and libclangReplace.

For now, functionality still exists in clang::replace namespace. Header guards
and file comments updated.

Differential Revision: http://llvm-reviews.chandlerc.com/D1548