Driver: switch Windows to static RelocModel

Windows uses PE/COFF which is inherently position independent.  The use
of the PIC model is unnecessary.  In fact, we would generate invalid
code using the ELF PIC model when PIC was enabled previously.  Now that
we no longer accept -fPIC and -fpoc, this switches the internal
representation to the static model to permit us to make PIC modules
invalid when targeting Windows.  This should not change the code
generation, only the internal state management.

llvm-svn: 290569