Restructure the propagation of -fPIC/-fPIE.

The PIC and PIE levels are not independent. In fact, if PIE is defined
it is always the same as PIC.

This is clear in the driver where ParsePICArgs returns a PIC level and
a IsPIE boolean. Unfortunately that is currently lost and we pass two
redundant levels down the pipeline.

This patch keeps a bool and a PIC level all the way down to codegen.

llvm-svn: 273566