[lldb/API] Add ability to check if module is backed by a file on disk

This patch introduces a new SBAPI method: `SBModule::IsFileBacked`

As the name suggests, it tells the user if the module's object file is
on disk or in memory.

rdar://68538278

Differential Revision: https://reviews.llvm.org/D118261



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>