PIC should not be enabled by default on Darwin with -static.

r245667 changed -static so that it doesn't override an explicit -fPIC
option, but -static should still change the default for Darwin for -fno-PIC.
This matches longstanding GCC and Clang behavior on Darwin and changing it
would be disruptive, with no significant benefit.
http://reviews.llvm.org/D15455
rdar://problem/23811045

llvm-svn: 256026