clang-rename: add a -export-fixes option

Use case: a class is declared in a header, and defined in two
translation units. clang-rename is asked to rename a class member that's
referenced in both translation units.

Using -i is not possible, as in case the first clang-rename invocation
touches the header, the second invocation will result in compilation
errors. Using -export-fixes handles this situation, each invocation can
work on the original source, and at the end the user can apply the
replacements with clang-apply-replacements.

Reviewers: klimek

Differential Revision: http://reviews.llvm.org/D21676

llvm-svn: 273910