Start converting usages of off_t to other types.

off_t is a type which is used for file offsets.  Even more
specifically, it is only used by a limited number of C APIs that
deal with files.  Any usage of off_t where the variable is not
intended to be used with one of these APIs is a bug, by definition.

This patch corrects some easy mis-uses of off_t, generally by
converting them to lldb::offset_t, but sometimes by using other
types such as size_t, when appropriate.

The use of off_t to represent these offsets has worked fine in
practice on linux-y platforms, since we used _FILE_OFFSET_64 to
guarantee that off_t was a uint64.  On Windows, however,
_FILE_OFFSET_64 is unrecognized, and off_t will always be 32-bit.
So the usage of off_t on Windows actually leads to legitimate bugs.

Reviewed by: Greg Clayton

Differential Revision: http://reviews.llvm.org/D4358