Move FileSystem functions out of Host and into their own classes.

More specifically, this change can be summarized as follows:
1) Makes an lldbHostPosix library which contains code common to
   all posix platforms.
2) Creates Host/FileSystem.h which defines a common FileSystem
   interface.
3) Implements FileSystem.h in Host/windows and Host/posix.
4) Creates Host/FileCache.h, implemented in Host/common, which
   defines a class useful for storing handles to open files needed
   by the debugger.

Differential Revision: http://reviews.llvm.org/D4889