add some infrastructure for codegen'ing complex numbers. implement addition

of complex.  We now produce optimal code for:

void test(_Complex float *Y) {
  *Y = *Y + *Y;
}

$ clang -emit-llvm cg.c | llvm-as | opt -std-compile-opts | llc -march=x86-64
...
_test:
        movss (%rdi), %xmm0
        addss %xmm0, %xmm0
        movss 4(%rdi), %xmm1
        movss %xmm0, (%rdi)
        addss %xmm1, %xmm1
        movss %xmm1, 4(%rdi)
        ret

llvm-svn: 39673