Stop treating -static as overriding -fPIC: they are distinct.

For some reason, clang had been treating a command like:
 clang -static -fPIC foo.c
as if it should be compiled without the PIC relocation model.

This was incorrect: -static should be affecting only the linking
model, and -fPIC only the compilation.

This new behavior also matches GCC.

This is a follow-up from a review comment on r245447.

Differential Revision: http://reviews.llvm.org/D12208

llvm-svn: 245667