[Driver] Add method to redirect output of Compilation.

Summary:
Currently output of child process, however in my use case, it
needs to be captured and presented to the user.

Add Redirect method to Compilation and use existing infrastructure
for redirecting output of commands.

Reviewers: tstellarAMD

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D21224

llvm-svn: 273997