Refactor FileAction out of ProcessLaunchInfo.

FileAction was previously a nested class in ProcessLaunchInfo.
This led to some unfortunate style consequences, such as requiring
the AddPosixSpawnFileAction() funciton to be defined in the Target
layer, instead of the more appropriate Host layer.  This patch
makes FileAction its own independent class in the Target layer,
and then moves AddPosixSpawnFileAction() into Host as a result.

Differential Revision: http://reviews.llvm.org/D4877