Convert getFileOffset to getOffset and move it to its only user.

We normally don't drop functions from the C API's, but in this case I think we
can:

* The old implementation of getFileOffset was fairly broken
* The introduction of LLVMGetSymbolFileOffset was itself a C api breaking
  change as it removed LLVMGetSymbolOffset.
* It is an incredibly specialized use case. The only reason MCJIT needs it is
  because of its odd position of being a dynamic linker of .o files.

llvm-svn: 206750